RECORD: MacCulloch, John. 1820. On peat. Edinburgh Philosophical Journal 2: 40-59, 202-217.

REVISION HISTORY: Transcribed (single key) by AEL Data 4.2013. RN1

NOTE: This work formed part of the Beagle library. The Beagle Library project has been generously supported by a Singapore Ministry of Education Academic Research Fund Tier 1 grant and Charles Darwin University and the Charles Darwin University Foundation, Northern Territory, Australia.


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ART. VIII.—On Peat. By JOHN MACCULLOCH, M. D. M. G. S. Lecturer on Chemistry to the Board of Ordnance, &c. Communicated by the Author.

SO much has already been written on this interesting substance, that it is scarcely possible at the present day to add any thing to the history of its origin, or to the description of its various appearances. The natural history of peat has perhaps, on the contrary, been occasionally rendered somewhat obscure by superfluity of discussion, while its chemical nature has either been misapprehended, or involved in mystery, from causes which need not here be noticed, but which will be sufficiently apparent to those who may be inclined to read all that has been written on this subject. It does not appear difficult to place the latter in a more intelligible light, by recurring to a few simple principles, in which some interesting analogies are also involved; while the former may perhaps also be simplified by a condensation and arrangement of the facts which are obvious to the examination of all, and which have been more or less accurately described by numerous observers. A sketch of the natural history of this substance is at any rate an indispensable preliminary to the few observations which it is here proposed to offer on its chemical nature.

The concurrence of a certain degree of moisture, and of a moderate temperature, appears necessary to regulate the decomposition of vegetables, so as to lead to the formation of peat. But the first condition admits of great latitude, as it is formed not only under water, but on drained declivities, in situations and climates that are subject to a very-moderate proportion of rain. It is also produced under a great range of temperature; but the observations hitherto recorded, have not enabled us to determine at what degree of heat it ceases to be generated. It is merely known, that in warm climates the decomposition of vegetables is so rapid, as to prevent its formation; and that, in these, it is therefore found only at high elevations, while, in the cold and temperate climates, it abounds even at the level of the sea.

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The quality of peat varies principally according to the different situations in which it is formed, as these differ in the degree of drainage and of temperature, and in the nature of the vegetables which they produce. Hence agriculturists have distinguished several varieties, which have, however, been sometimes also named from differences of texture, at others from differences of situation. The former distinctions have been founded either on the more or less perfected state of the process, or on accidental circumstances, which will hereafter be apparent. The whole of these varieties may be included in the following divisions: namely, Mountain, Marsh, Forest, Lake, and Marine, Peat. To these ought to be added transported peat, which, although of very partial occurrence in this country, appears to abound in Holland, and in other parts of the continent of Europe.

These several essential varieties occasionally present many inferior distinctions of quality and aspect, arising either from the nature of the plants which have contributed to their formation; from the varying extent to which the process has been carried; from intermixture of character, dependent on the variable state of drainage or other causes, or from the alternation or admixture of transported earth or of shells. In some instances, even in situations very different, they approach so nearly in character as to be undistinguishable; and this happens more particularly in the peat of lakes and in that of marshes, and in those cases where the process is most complete.

In general, peat forms a single superficial stratum, lying either on the solid rock, as in the islands of North and South Uist, Lewis, and the remainder of that chain, or on the alluvial soil which, in other places, is found above the fundamental rock. In other cases it is found to alternate with sand and gravel, with clay, with shell marl; while, in a few instances, it is found at considerable depths, buried under a thick bed of clay, or of other alluvial soil. The alternations of sand and gravel occur either on sea-shores, at the estuaries of rivers, or their termination in lakes, or in other situations where large quantities of these materials are carried down by rivers, so as for a time to cover the plain, and destroy the process of vegetation. In those cases where large deltas are formed or bays are filled up, the deposition of earth continues to proceed, till the

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original peat becomes buried to a considerable depth, and as such plains are generally brought into cultivation, it is never again renewed. Alternations or, deposits of marl are only found in those situations where the peat has been generated in a lake; and they offer a criterion, not only of the former existence a lakes which have long since disappeared, but are capable of determining, to a certain degree, the depth which these once possessed.

Mountain peat is not limited strictly to mountainous declivities, but is found in all situations where the drainage is considerable. This variety is commonly loose in texture, and its average thickness may be estimated at a foot. It rarely amounts to two feet, and often does not exceed as many inches. As an article of fuel it is of little value, when compared to the other varieties, from its thinness and want of compactness; but it is more interesting in an agricultural view, from the great extent of surface which it occupies in Scotland, and from the comparative facility with which, in low situations, it is brought into cultivation. The dry moor lands are generally covered with this variety, or with a variety intermediate between it and the more compact peat of marshy and ill drained situations. The plants which principally conduce to the formation of mountain peat are the following:

Erica cinerea. Polytrichum commune.
vulgaris. Lycopodium clavatum.
tetralix. Lichen rangiferinus.
Myrica gale. Nardus stricta.
Empetrum nigrum. Scirpus cæspitosus.
Tormentilla erecta. Juncus squarrosus, together with various grasses, chiefly belonging to the genera Airs, Agrostis, and Carex.
Arbutus uva ursi.
Vaccinium vitis idæa.
Juniperus communis.

Marsh peat is chiefly formed in flat rained lands, or on the margins of lakes, while it is also produced during the last stage of the formation of lake peat, with which it is therefore often found united, constituting the upper surface of the more compact variety which has been generated under the water. It is often found occupying extensive plains, and presents a great variety of aspect. These varieties depend chiefly on the very various state of drainage, arising either from situation, or from difference of season; and as such extensive plains often retain

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water very unequally, the same tract will often be found to present considerable diversity of character. In some situations it is semifluid beneath, and covered with a matting of tenacious and half decomposed vegetables, or else it is interspersed with partial spots of the same nature, maintained by the firmness of the heath, or of the rushes which have established themselves in those places. In other situations it is intermixed in minute patches with mountain peat, according to peculiarities in the form of the surface, capable of determining the formation of these different varieties.

Marsh peat occasionally forms a very imperfect stratum only, the roots of the rushes and grasses retaining much of their original firmness, in which case it is sometimes known by the name of Rush-turf, while, in other cases, a stratum of perfect peat is found beneath it; an occurrence which takes place when a trembling bog has been accidentally drained; or when, by the gradual accumulation of the peaty soil, the level has been so far raised as to cause a constant and natural drainage. It is in this species of peat that the Sphagnum palustre forms so conspicuous an ingredient, and this plant is indeed, from its nature, limited to the peculiar situations in which marsh-peat is formed. The other plants, which chiefly contribute to its formation, are included in the following list:

Erica valgaris, Airs cæspitosa.
tetralix. Festuca fluitans.
Myrica gale. Caltha palustris.
Vaccinium oxycoccos. Hydrocotyle vulgaris.
Eriophorum polystachium. Lysimachia tenella.
vaginatum. Menyanthes trifoliate.
Schœnus albus, and others of this genus. Ranunculus flammula.
Comarum palustre.
Scirpus cæspitosus, and others of this genus. Narthecium ossifragum.
Pinguicula vulgaris.
Pedicularis palustris. Drosera longifolia.
sylvatica. anglica.
Orchis maculate. rotundifolia.
conopsea. Triglochin palustre.
Juncus, various species. Phalaris arundinacea.
Carex, various species. Arundo phragmites.
Aira aquatica. Some of the Equiseta.

The formation of Lake peat is of a much more complicated nature than that of the two preceding kinds, and is moreover rendered additionally interesting, by its connection with that im-

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portant set of changes on the surface, which consists in the obliteration of lakes, and the increase of dry land. From the remarks immediately preceding, it will appear, that it is intimately connected with the generation of marsh-peat; since, during its whole progress, it is attended by, and finally terminates in this variety.

There are thus two distinct modes in which peat is formed in lakes, and, in each, a distinct set of plants is engaged. There is, however, a certain point at which these unite, and at which both processes are combined in producing the result by which the water is ultimately excluded, and a mass of solid materials is at length found occupying the place which it once possessed.

In the shallow parts of lakes are found numerous subaquatic plants, which, in summer, flower at the surface, subsiding to the bottom after that process is completed. By the annual death of portions of these, a stratum of peat is formed at the bottom of the water, where this process is also constantly going on by the successive decomposition of the Scirpus acicularis, Lobelia dortmanna, Subularia aquatics, and other plants, which form a continuous mat of verdure, where the water is sufficiently shallow to admit of their growth. The death of fresh-water shell-fish, and the deposits of earth and sand brought into the lake by rivers, assist still farther in shoaling the water, and in thus affording a lodgment to another class of plants, which advance from the margin in succession, according to their several propensities to thrive in greater or less depths, or on the half drained and marshy meadow by which the lake is bounded.

In other cases, no distinct bed of peat is produced from the subaquatic plants, but the whole process is the result of a gradual extension of the margin. This may readily be examined in those lakes of which the interior declivity is gentle, and at those points where deltas are formed by the entrance of streams. The growth of Arundo phragmites, Scirpus lacustris, Lobelia dortmanna, and other plants, here serves to detain much of the sand and earth which would otherwise be carried forward into the deeper parts of the lake, and thus gradually affords a place first to the rushes and grasses which flourish in inundated soils, and lastly, to the various plants which, in succession, grow in marshy grounds of different degrees of moisture.

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Thus a stratum of peat is gradually formed from the original margin inwards, covering, by its extension, the bottom of the lake, till, in certain cases, the water is entirely excluded, and the whole becomes a peat-bog, or, as may happen in other circumstances, a meadow. Under such a stratum of peat, the shells which have been deposited in the course of years, are at length consolidated into the marl beds, formerly noticed, which are so frequent under the flat peat-bogs of the Highlands.

These are often of such an extent and thickness, as to form valuable quarries, if this name may be adopted, for the purposes of agriculture; the calcareous manure which they afford, being of the best quality. Many mossy lands have thus acquired a value which was little foreseen, and by which the proprietors or purchasers have very unexpectedly profited. The discovery of these marl-pits has generally been the result of accident, or of the excavations used for procuring fuel; very few of the farmers or proprietors being aware of the true nature and origin of these deposits. A more general diffusion of that knowledge among them, would lead to the discovery of marl in many parts of the Highlands, where it is as yet unknown, and where calcareous manure is, from its rarity and distance, extremely valuable. As the shell-fish, which contribute to its formation, exist even in very small collections of water, it may often be found in the mountain peat-mosses of very limited extent, as well as in the larger, which have resulted from the partial or total obliteration of considerable lakes. It may be sought for with a prospect of success, in all those mosses which are bounded by rocky or elevated margins, or which occupy basin-shaped cavities. Some of these still give passage to the river which once flowed into the original lake, and which has, by its deposits of earth, contributed to fill the cavity; while others, which give passage to no river, have been produced by the gradual increase of vegetation from the margin in the mode just described. Experience in the forms and disposition of existing lakes and pools, will easily guide the eye in the selection of those spots where marly deposits may occasionally be expected. It must, however, be added, that marl is sometimes found on declivities, mixed with, and buried under mountain-peat. But these very singular deposits are rare, and the produce of terrestrial

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shells, of which an account was given by the writer of this article in a paper on Glen Tilt, published in the Geological Society's Transactions.

It is easy to see how lake-peat may alternate with. strata of clay, and of gravel and sand. These have been the result, as at the estuaries of rivers before noticed, of occasional floods, carrying with them so large a quantity of earth as to suffocate and destroy for a time the process of vegetation; and it is easy to conceive how the occasional addition of such transported materials, might, for a certain time, impede the re-establishment of a vegetating surface, so as to form an earthy stratum of considerable depth. It is also easy to understand how the gradual addition of earth, in a quantity insufficient to destroy vegetation, may, instead of peat, lead to the production of an ordinary vegetable soil, and how the margin of a lake, in the act of filling, will, instead of a peat-bog, become a fertile meadow.

In concluding this sketch of the formation of lake-peat, it is interesting to observe the simplicity and certainty of the processes by which water is converted into land in the interior of a country, as it is near the marine estuaries of rivers.

The plants which chiefly contribute to the formation of this kind of peat, are the following, and they are separated, in some, measure, according to the regions which they occupy in the water or on its margin.

Conferva bullosa. Arundo phragmites.
Lemna minor. Scirpus lacustris.
trisulca. Lobelia dortmanna.
Nymphæa alba. Scirpus acicularis.
lutea. setaceus; and others.
Potamogeton natans, Subularia aquatica.
heterophyllum; Isoetes lacustris.
and others of this genus. Alisma plantago.
Chata vulgaris. ranunculoides; and
Hippuris vulgaris. Others.
Callitriche verna. Sagittaria sagittifolia.
autumnalis. Butomus umbellatus.
Myriophyllum spicatum. Acorus calamus.
verticillatum. Phalaris arundinacea.
Ceratophyllum demersum. Poa aquatica.
Utricularia vulgaris. Juncus conglomeratus.
minor. effusus; and others.
Sparganium natans. Schœnus mariscus; and others.
erectum. Menyanthes trifoliata.
Ranunculus aquatilis. Comarum palustre.
Hydrocharis morsus ranæ. Some Equiseta.

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It is unnecessary to add to this enumeration the Sphagnum palustre, and some other plants which were before mentioned under the head of marsh-peat.

The next division is that of Forest-peat. It has often been asserted, that all peat was the produce of decayed woods; but the preceding remarks render it unnecessary to enter into a formal discussion on the unfounded nature of that opinion. Even those peat-bogs which now cover the decayed trunks and roots of trees, and which have been supposed to have originated in their destruction, owe great part of their bulk to the growth and decay of plants subsequent to the period of their fall, and are now increasing by a continuation of that process which forms the peat of marshy or of wet moor land. During the growth of a forest, in favourable situations, a stratum of peat is formed by the accumulation of decaying leaves and branches, and by the death and renovation of the plants which grow under their shade. Where, in the lapse of time, the trees of these forests have fallen, whether from the effects of age, from storms, or from other causes, a certain portion of the wood decays, and adds to the original deposit; while the growth of additional plants of different kinds, perpetuates the process. In these cases, the smaller branches, and even the larger, being exposed for a longer period to the effects of air and water united, and being, from their size, more susceptible of decomposition, are ultimately converted into peat; while, in a few situations, even the trunks disappear. Where, on the contrary, the growth of the peat stratum bas been so rapid as to cover the trunks, so as to exclude the alternate action of air and water, these remain for a long period unchanged, and are often dug up unaltered, so as even to be applicable to the ordinary purposes of timber. Oak, in such circumstances, is well known to acquire a black colour, and to become valuable for the purposes of the cabinet-maker. As the roots are soonest and most effectually protected from the causes of decay, they are generally found unchanged, even when the trunks have entirely mouldered away. It has been supposed that peat possessed some mysterious antiseptic properties, by which this effect was produced; but it differs in no respect from that wish equally occurs where wood is so far buried, as in the case of piles, as to be excluded from the influence of the air, or from alternations of dryness and moisture.

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Although the submerged wood of peat-bogs thus continues often unchanged for a long period, in other cases it undergoes a chemical change, without loss of texture, and acquires the peculiar properties of peat. It will hereafter be shewn, that a longer continuance Of the same action of water produces an incipient bituminization, and that time alone is probably required to convert such wood into lignite, or into a substance resembling brown coal, surturbrand, or even jet; products, of which the peculiarities are in some measure owing to collateral circumstances, which it is unnecessary at present to consider.

It is scarcely necessary, in concluding this subject, to remark, that forest-peat must vary materially, according to the proportions in which wood and the ordinary marsh plants enter into its composition, independently of the numerous other circumstances by which the characters of every variety of this substance are modified.

The trees which in Scotland enter into the formation of forest-peat, are chiefly oak, fir, alder, and birch. To these may be added, as more rare, the ash, the roan, the hazel, and different species of willow. To enumerate the plants which grow under their shade, or occupy the soil on which they have fallen, would, with a few trifling additions, be to repeat much of the catalogues which have already been given.

It has scarcely been noticed by our writers on this subject, that peat is also occasionally formed on sea-shores, by the death and renewal, in salt-marshes, or in other analogous spots, of those plants which affect maritime situations. Such peat may, however, be found on the shores of the maritime Highlands, and in other analogous places, and it often forms strata alternating with sand, or with the muddy deposits of rivers. It is not uncommon, in those places where the estuary of a considerable river is a sea-loch, and where, by the gradual extension of the shallow shores, from the same causes which operate in shoaling inland lakes, the sea becomes gradually excluded. West Loch Tarbet and Isla, in this country, present, among other places, very accessible and conspicuous examples of this process, in its several stages. In the former spot, it may be seen in a State of great activity, and a considerable accession of land is thus gradually taking place. In Isla, it has long since terminated in the flat

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tract by which Loch Greinord communicates with the head of Loch-in-daal, and the stratum of maritime peat is here submerged beneath a considerable depth of alluvial soil.

In North Uist, peat is found in analogous situations, appearing, in these cases, to have been generated by those plants which grow in sandy soils, and which have, after a certain time, been overwhelmed by the accumulation of blown sand. The submergence of maritime peat by soil, where the estuary of a river is the sea, is so exactly analogous to that which takes place where a river terminates in a lake, that it is unnecessary to dwell on it. It is equally obvious, that the power which the semi-maritime plants exert in detaining the alluvia of rivers, or the sand of the sea-shore, is similar to that which the common reed and bulrush exert on the shores of lakes, and it is therefore also unnecessary to dwell on this subject. As the marl of fresh-water shells is found alternating with inland peat, so deposites of sea-shells, in various states, are found intermixed with that which is of maritime origin.

The peat formed on sea-shores by the plants that grow in sand, is small in quantity; nor is it always certain that it originates in these plants, even where it is difficult to discover to what others it can have owed its origin. These plants are chiefly the following:

Triticum junceum. Valantia cruciata.
Elymus arenarius. Thalictrum minus.
Arundo arenaria. Trifolium repens; and various well known grasses.
Carex arenaria.
Galium verum.

The plants which grow on the alluvial flats, or in the salt-marshes, are more numerous, and comprise principally the following species:

Glaux maritima. Scirpus triqueter.
Juncus maritimus. Matricaria maritima.
Triglochin maritimum. Statice limonium.
Aster tripolium. armeria.
Schœnus mariscus. Artemisia maritima.
Arenaria peploides. Bunias cakile.
marina. Chenopodium maritimum.
Scirpus holoschœnus. Salicornia herbacea.
maritimus. fruticulosa.

But the most conspicuous and important plant in extending the alluvia of maritime estuaries, is the Zoatera marina, which forms

VOL. II. NO. 3. JANUARY 1820. D

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large submarine plains at considerable depths under the sea, and, by detaining the sand and mud, gradually raises the bottom so far above the high-water mark, as to enable it to afford a place for the plants of the preceding catalogue. By the bulk and extent of its roots, it also serves to form the principal part of the peat stratum which is found on sea-shores.

The several varieties above enumerated, include all the depositions of peat which are found in their natural situations; but another variety remains, which was already mentioned, and which it is necessary to notice, before quitting this part of the subject. It consists of the finer parts of perfect peat, which have been transported by water to cavities capable of detaining them. This transported peat is rare in Scotland, and generally occupies small patches in mountainous land, where it produces a liquid or trembling bog: on the Continent of Europe, it appears to form more extensive tracts, and to be of considerable importance. When drained, it constitutes one of the most solid and perfect varieties of peat.

The progress of change from the living vegetable to peat, is most easily traced in the flat mosses of plains, and may be examined in the deep sections which are made for procuring fuel. In these, a regular gradation may be traced, from the spongy half-decomposed vegetable matter to the solid inorganic peat, in which nearly all traces of the plants have been obliterated. A similar gradation, but less perfect, may be found in mountain-peat; but the progress of the other varieties is, from their peculiar nature, less susceptible of examination. Where the living plants terminate, their roots are found intermixed with the half-decomposed matter, produced by their leaves and fragments, forming a spongy mass. Lower down in the stratum, the pulverized peat is seen mixed with a few fibres or fragments still resisting decomposition. These gradually disappear in the progress downwards, until at length a finely powdered substance alone is found; the process being completed to the eye in the total destruction of all the organized bodies. When this process has occurred on a drained declivity, the pure peat forms the powdery heath-soil used by gardeners for rearing certain plants; where, on the contrary, the soil is wet or inundated, the Powder is intermixed with water, which, on being drained,

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leaves it in a state often highly compacted and firm, producing that peat which is more valuable for the purposes of burning. A variety of modifications result from collateral circumstances or intermediate conditions; the chief of which are, differences in the nature of the vegetables, in the time during which the process has lasted, in the degree of drainage, and its occasional variations, and in the elevation, or other causes, affecting the temperature, or the moisture of the atmosphere.

The commencement of the process is easily traced in the Sphagnum paluatre, which, in some situations, forms an important ingredient in peat. In this plant, as the lower extremity dies and is decomposed, the upper, like many other mosses, sends forth fresh roots, and extends its offsets; the individual becoming thus in a manner immortal, and supplying a perpetual fund of decomposing vegetable matter. A similar process, although less distinct, takes place in many of the rushes and grasses, the ancient roots and leaves dying, while the existence of the plant is perpetuated by the annual renovation of both. Many other vegetables add to the common stock, by their annual death, their existence being repeated in seedling plants; while others, of a perennial nature, contribute only by the ordinary decay and renewal of, their leaves.

It is therefore evident, that the growth of peat is constant, but that it depends, at the same time, on the continuous growth of the vegetable covering. Hence mountain-peat is always increasing while it continues to bear those plants from which it was originally produced; as its state of drainage, and looseness of texture, are such as never to refuse a hold to fresh plants of the same nature, even after the vegetating covering has been completely removed. There are certain cases, however, in which the growth of mountain-peat ceases, the vegetating covering being replaced by plants which, under the circumstances that first produced them, continue to grow, and possess no tendency to form that. substance. These cases are interesting to the agriculturist, as they form, in fact, the several processes by which this species of peat is converted into green pasture or into arable land. The pasturage of sheep, the application of calcareous manures, or of clay or sand, burning, and the turning of the surface by agricultural implements, are the chief means by

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which the vegetation of these soils is exchanged for more profitable plants. To these must be added the growth of larch, under which the original covering is gradually extirpated, and replaced by a green and grassy surface, applicable to the pasturage of cattle.

Although the formation of marsh-peat ceases, when, from the gradual increase of elevation, or from other causes, the bog becomes drained, this substance still continues to be generated, but in a slower manner, by the plants that form it on mountain-declivities.

The formation of forest-peat must be considered to have terminated with the decomposition of the wood which has first fallen, although the process still goes on in a different manner, by the growth and decay of the various plants which occupy the same place. A few cases are however known, where it appears that forests have successively grown and fallen on the same spot, so as to have continued the original process.

The peat of lakes is necessarily completed, when once the water has disappeared; and in a similar manner, that which is formed from maritime plants, ceases to grow, when, by the final exclusion of the sea, the character of the vegetation is changed.

Of the formation of transported peat there is no end, as long as the flow of water from mosses continues, and cavities exist in which the material can be deposited.

As the increase of peat necessarily keeps pace with the growth of the plants from which it is formed, it is evident, that the cessation of the one is implied in that of the other, with the exception of transported peat, which does not directly depend on the same cause. That it may be renovated after cutting, it is therefore necessary that the process of vegetation be renewed where it has thus been suspended. In mountain-peat, it has already been shewn that this takes place naturally, as this variety generally approaches so much towards ordinary vegetable soil, as readily to admit of the lodgment and growth of seeds. But the compact and perfect varieties which are exposed by the operation of cutting, are not thus susceptible of renewed vegetation, as no plant seems willingly to attach itself to peat in this state. But if the pit has been so formed as to admit of the stagnation of water, the process is sometimes renewed by the same series

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of operations which takes place in lakes, and thus the cavity becomes once more filled with the same material, always, however, distinguishable for a long period by the laxity of its texture. If the exposed surface is drained, no such event follows, but the spot appears to remain for ever naked, and a course incapable of augmentation or renewal, as may daily be witnessed in mountains, where the perfectly formed and compact peat has been either naturally or artificially exposed. This is now well understood; and it has accordingly become a general practice to reserve the superficial covering, and replace it on the bottom, after the peat required for fuel has been removed.

The time required for the production of a given depth of peat has been a frequent subject of inquiry. In some cases, its growth has been found so rapid as to be sensible to the observation of individuals; but, in general, it has been necessary to have recourse to evidence founded on circumstances involving a much longer period of time than human life. Such registers of the date of given depths of peat have been found in works of art, or in the marks of human industry which have been submerged in it, and of which the time admitted of being ascertained within certain limits. From these several modes of judging, it has appeared, that the time required for the formation of a given depth of this substance, is so various, that no general conclusion can be drawn on the subject. It has been remarked on the Continent of Europe, that cavities of seven feet in depth have been filled with it in the short space of thirty years. This occurrence is, however, limited to partial spots, and depends on a combination of favourable circumstances, which must be considered rare. It only takes place in the smaller excavations, in which the vegetation of aquatic and marsh plants is unusually luxuriant. Two cases have often been quoted, of peat-mosses formed on fallen forests, the one at Drumlanrig, the other near Loch Broom, the former of which was completed in sixty, the latter in fifty years; but as the depth is not specified, no conclusion can be drawn from these instances. In innumerable other instances, Roman works of art, and traces of Roman roads, have been found beneath peat-mosses, but the depth has seldom been measured; so that from these also, no conclusion respecting the quantity produced in a given time, can be drawn.

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In a few cases, however, the depth has been measured. A coin of the Emperor Gordian, is quoted as having been discovered at thirty feet in depth; and other Roman coins are mentioned as having been found in Italy even at that of fifty feet. From the latter, of which the dates are not given, the time required for the production of this quantity of peat, can only be conjectured within very wide limits. In this country, Roman utensils, and traces of Roman roads, have been found at the depth of eight feet; but as other parts of the moss in the spot in question attain to the depth of twenty feet and upwards, the quantity produced within the time ascertained by this register, may be considerably greater. Recently, the palings of a park described by Camden, have been discovered beneath Chat Moss; but the depth of superincumbent peat in this place has not been mentioned.

It appears unnecessary to be anxious on this subject, as, from a variety of circumstances, it must appear evident that no general conclusions can be drawn. The chief causes which influence the growth of peat, are situation, temperature, and the degree of moisture, all of them being causes which affect the rapidity of vegetation. In those cases where mountain-peat is in contact with the solid rock, it is obvious, that its growth, particularly in the first stage, must be very slow, as the time which the lichens and mosses require to lay the foundation of a vegetating surface is considerable. In all cases, the peat of drained declivities is of tedious growth, and, as was formerly mentioned, rarely attains a greater depth than two feet. The production of marsh and of lake peat, are on the contrary rapid, and that in proportion to the circumstances of various kinds, by which the luxuriance of vegetation in these situations is promoted. That of forest-peat must, in most cases, be even more considerable, as a sudden accession of depth is produced by the fall of the trees which contribute to its formation. Many of the deeper mosses of all countries, appear accordingly to owe a large portion of their bulk to this cause, and are found in places well known by historical record, as well as by internal evidence, to have been once occupied by forests.

Where no registers of time, derived from submerged works of art, or from historical record, can be discovered, the antiquity

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of any given mass of peat may be presumed from other circumstances. These are, chiefly, its alternation with beds of clay or alluvium, and the degree in which it approximates in its chemical qualities to brown coal or bituminized wood. The latter change requires a length of time greater than that during which the greater number of the superficial peat-mosses of this country has existed; and, in most cases, the alluvia which have been generated by the deposits of rivers are evidently of a very high antiquity.

Under the various circumstances in which peat is found, it exists, as a soil, in various states, namely, in that of a fibrous and spungy mass, a powdery substance, a compact body, and a semifluid paste. The former condition occurs in all cases where the process has not been perfected, and presents infinite varieties. The second state is peculiar to mountain or drained peat. Compact peat, again, forms the predominant variety, in Scotland at least, as far as the mere bulk of the deposit is concerned; although the drier kinds occupy a much wider extent of surface. The semifluid state occurs, not only in transported peat, but in other cases where it has been formed in lakes or marshes, which, from their forms, have the power of retaining water. Time consolidates even this peat, by filling the cavity with solid matter, and thus gradually excluding the water; but it may continue for a long period in that state, as it does peculiarly on the continent of Europe. The well known inundations of fluid peat, have arisen in these particular cases, from the failure of the barrier by which such a cavity was bounded.

It is not within the limits of this paper, to notice the capacity of these several kinds of peat for receiving agricultural improvement, nor the various well known methods by which they may be brought into cultivation. It must, however, be obvious, that no general rules can be applicable to the whole, and that the great diversity of qualities possessed by this substance, indicates corresponding differences of treatment. It is also evident, from watching the natural processes by which peat is converted into soil, that pulverization forms the essential part of every mode of treatment. In draining a compact peat, art thus imitates what nature effects on mountain declivities; and, in mixing earths with it, the same results follow, which, in nature,

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occur from the dispersion of sand in the neighbourhood of the sea, from the intermixture of clay and sand near the estuaries of rivers, and from other more tedious actions by which its texture is loosened, and maintained in a lax state. It is in these cases that peat becomes naturally susceptible of cultivation with comparative ease; and it is by attending to these circumstances, that the agriculturist must be directed in the choice of subjects for his improvement, wherever a choice is in his power. A consideration of the effects produced by the application of calcareous manures, by burning, and by other means, must be left to those under whose peculiar province this subject falls.

The brown colour of mountain-streams evinces the partial solubility of peat in cold water, or at least proves that the mass in question contains a soluble ingredient. The quantity of matter thus held in solution is however small, and seems only to be extracted during the incipient or incomplete stages of the process. Peat, of which the formation is perfected, appears to communicate no stain to cold water, although boiling water still extracts a portion of the same ingredients. Water thus impregnated has been supposed to possess some peculiar antiseptic powers, the existence of which is, at best, doubtful, and which, if they do exist, appear to depend on accidental impregnations, not on the peculiar hydro-carbonaceous compound which communicates the brown colour. The obscurities which have attended this subject, appear to have arisen from inattention to the various nature of the substances which may be, and occasionally are, dissolved by waters flowing over peat, or stagnating in its cavities. One of these must be considered of an accidental nature, and it is the tanning principle extracted from the undecomposed bark and roots of various vegetables, and, among others, from the Tormentilla erecta. It is probable that, where animal bodies have been preserved in a state of integrity in peat water, this tanning process has been the cause; although, in certain cases, the constant application of mere water is capable of preventing the putrefaction of the muscular and membranous parts of animals, by converting them into adipocire. Peculiar effects are also produced by ferruginous solutions, which must also be considered as accidental, and which appear to arise from the conversion of pyrites into sulphate of iron. It is not impro-

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bable that, where tan is held in solution, a portion of gallic acid may also occasionally be dissolved; but it is nearly impossible to put these subjects to the test of direct experiment, from the difficulty of procuring solutions of peat in a state fit for such accurate examination. The acid which may be extracted from peat, by decoction in water, is small in quantity, and appears rather to be generated by the same process which converts the vegetable fibre into peat. It will hereafter be seen, that when vegetable matter is exposed to heat, the carbon and hydrogen enter, together with portions of the other vegetable elements, into new compounds, some of which are partially soluble in water. A familiar instance of this occurs in the roasting of coffee. During this process a portion of acid is also generated, which, on pushing the process to extremity, forms a conspicuous part of the result of the destructive distillation of vegetable matter. The action of water on vegetables, in the case of peat, as in that of manure, produces hydro-carbonaceous compounds of an analogous nature, some of which are similarly soluble in water, while others form insoluble peat; and it appears a necessary consequence that, in this process also, acetic acid should be generated. The acid extracted from peat is in fact acetic acid, but the time required to conduct a direct experiment of this nature, so as to produce a decided result, is such as to prevent the possibility of instituting any for that purpose.

The soluble hydro-carbonaceous compound which communicates the brown colour to peat water, resembles, when obtained in a state of purity, that which is generated by the decomposition of manure, (abstracting the salts which abound in this,) as well as that which is produced by roasting coffee, or by the application of a regulated heat to other vegetable matters. It is like that also which, in highly torrified malt, or in sugar, communicates the brown colour to malt liquors. It is probable that, in all these cases, this compound is subject to considerable variations; but chemistry as yet offers no certain methods of accurately distinguishing these. Distillation, which separates the hydrogen and other volatile matters from the carbon, proves differences, the exact degree of which it does not enable us easily to appreciate.

This compound, as it is deposited naturally from peat waters, is sometimes found in bogs in such quantity as to admit of ready

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examination. Occasionally, it is dispersed among the more solid peat, which consists of powdered materials, fragments and fibres, to which it communicates a black colour and pitchy appearance. It cannot then be extracted by water, as far as I have experienced, since, when once hardened, and in a detached state, it is not soluble even in boiling water. When first this matter is procured, it is soft, but when dried, it is brittle, and breaks with a splintery and conchoidal fracture, presenting a considerable resemblance to asphaltum. It also sometimes resembles that substance in lustre, but is at other times dull, and the colour is black. It is heavier than water, and is not electric. It is not acted on, as already remarked, by boiling water, nor is it soluble in alcohol, ether, or naphtha; in which latter circumstance its difference from asphaltum, which it otherwise so much resembles, is strongly marked. On distillation, it gives the same results as ordinary peat, namely, acetic acid and vegetable tar, the charcoal remaining behind. These results prove, that, like undecomposed vegetables and peat, it contains oxygen as well as hydrogen and carbon. The quantity procured for the experiments in question, was too small to show whether any azote exists together with the other elements, as no ammonia was found in the small proportion of acid obtained.

It has not, as far as I know, been observed by our numerous writers on this subject, that peat is occasionally luminous, although a remark of an analogous nature is made by some oriental traveller, whose name does not at this moment occur. I have often, however, witnessed this appearance, and most remarkably in the bared bogs of North and South Uist. The luminous matter is dispersed in small points over the brown surface, and as these are very minute, while the colour of the light is also blue, they are scarcely visible at a small distance, whence, probably, they have escaped notice. This fact is analogous to that which occurs during the decomposition of wood; but the present state of chemical analysis does not appear to offer any means of ascertaining the cause of the phosphorescence.

The specific gravity of peat varies considerably, and, in a great measure, in proportion to the degree of its perfection; in a few instances it is found to sink in water. Its qualities as a fuel depend much on its varying condition in this respect; that

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of North and South Uist, and of this chain of islands in general, is so compact as to afford a flaming fuel nearly equal to the inferior kinds of coal, while, after being once dried, it is again incapable of absorbing water.

(To be concluded in our next.)

ART. IX.—Account of a Peculiar Modification of Vision. By JOHN GILLIES, M.D. Member of the Wernerian Society. In a Letter to Dr Brewster.

DEAR SIR,

IN compliance with your request, I now send you an account of the singular modification of vision formerly mentioned to you, and of which I can find no similar instance on record. It occurred frequently to me during the year 1816. Since the third day of the following year, with but three exceptions, and these at very distant intervals, I have never again experienced any return of it. I have to regret much, that my observations on this subject have been rather limited and imperfect. On this account I have purposely deferred laying the particulars before the public, in the expectation that their recurrence might enable me to render them more complete; but of this there is now very little probability, when so long a period has elapsed since this affection has visited my eyes with any degree of frequency.

This modification of vision usually made its appearance in the following manner, without any previous intimation of its approach. When sitting in company, or otherwise engaged in any occupation not requiring continued or minute attention, all objects within the range of my vision, appeared to recede from me, and be proportionally diminished in magnitude. These appearances at first were generally so very slightly different from my natural vision, as to produce in my mind some doubt whether I had not been deceived in supposing there was any difference. Any doubt of this kind, was, however, soon removed by a gradual increase from one degree to another, till it had reached its maximum, which usually took place in the course of a few minutes from the commencement. In this condition, which

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THE

EDINBURGH
PHILOSOPHICAL JOURNAL.

ART. I.—On Peat. By JOHN MACCULLOCH, M. D. M. G. S. Lecturer on Chemistry to the Board of Ordnance, &c. Communicated by the Author. (Concluded from Page 59.)

IN describing the natural solution of Peat in water, a few of its chemical qualities were necessarily mentioned. Those which remain to be noticed are of little or no moment in a practical view, nor are they such as to throw much light on the chemical nature of this substance, or on that of the process by which vegetable matter is converted into peat. Alcohol, ether, and the alkaline lixivia, extract from it a brown matter, analogous to, or identical with, that which has already been described as capable of being dissolved by water. On evaporating the two former fluids, this matter is obtained in a solid form, but, in the latter case, it combines with the alkali, so as to form a saponaceous compound, analogous to that which is produced by dissolving bistre or wood-tar in the same solutions. It is unnecessary to describe the results obtained from the action of the essential oils, ammonia, or the mineral acids; as they are either analogous to the former, or resemble those which the same substances produce with unchanged vegetable matter, and throw no light on the subject.

The object to be kept in view in the chemical examination of peat, is to determine the nature of the change which has taken place between the destruction of the vegetable organization, and the conversion of its substance into this new compound. It is more particularly interesting, to discover in what respect peat approaches to the bitumens, or rather to the bituminated lig-

VOL. II. NO. 4. APRIL 1820. O

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nites; since, through this latter stage, it must pass in its transition from the vegetable to the mineral class of substances. This question is more or less remotely connected with a very interesting train of geological inquiries. Peat has in fact been confounded with the bituminated vegetables, and has been supposed to derive its inflammable quality from, the formation of bitumen during the process by which it has been generated. It will, however, be found, on a more strict investigation, that there is an essential distinction between common peat and bituminized wood, and that even when it does show some of the characteristic marks of the bitumens, the degree in which it has undergone this change is very limited. The Ince peat of Lancashire appears unquestionably to be accidentally penetrated by petroleum, derived from the usual source of a bituminous spring.

Although in a practical view, therefore, peat may be considered as an indefinite substance, and in a state of transition to bituminized vegetable matter, it is convenient for the present purpose to divide the process from the death of the vegetable to its bituminization, into two stages. The first of these comprises the change into mere peat, in which no traces of bitumen can be discovered, and the last may be conceived to extend from the time at which it first gives traces of this substance, to that in which the characters of peat have been replaced by those of bituminized wood. It will shortly be seen, that we are in possession of chemical means, by which pure bitumen can be distinguished from pure peat, or from that substance which constitutes its essence; and the application of the same powers enables us, although less perfectly, to distinguish that state to which alone the name of Peat should, for the sake of accuracy, be applied, from that in which the process of bituminization has commenced.

The powers of chemical analysis are as yet so limited, that we have no means of examining directly the differences of the various substances which are found throughout the whole of these changes, from the simple vegetable matter downwards; or, at least, the results which are obtained from the application of these means, are imperfect and unsatisfactory. But considerable light may be thrown on the subject, by that operation former-

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ly considered as an analysis, namely, the application of heat in close vessels, or destructive distillation. By conducting this process in the usual rude manner, there are obtained a set of new compounds, together with a portion of the elementary constituents from which these compounds, and the original subject, are formed. With more care, a new set of compounds alone are obtained, and, in either case, these are again capable of being subjected to a new analysis, so as to furnish, in a sufficiently accurate manner, the required information. It is unnecessary for the purposes of this paper, to enter into any minute details on this subject; but it will be sufficient to state, that by examining the various nature and proportions of these compounds, as they are obtained from the different materials, or, if necessary, by again decomposing them into their elements, the proportions of these elements, under different circumstances, may be obtained with sufficient accuracy for the objects in view. Thus the progress of change in the vegetable matter, may be traced by the gradation in these proportions during the progress of that change; and thus also certain useful analogies may be discovered between peat under all its forms, and other compounds which differ from it rather in their mechanical than in their chemical nature.

If fresh vegetable matter, (and it is here most convenient to consider the ligneous substance), be subjected to distillation by a naked fire, certain volatile products are obtained, and charcoal remains behind. The chief of these volatile products are Tar *, a volatile oil, and acetic acid. A varying proportion of

* I have given to this substance the name of Tar, because that name is so applied in the charcoal manufactories, where it is obtained in considerable quantity. The tar used for naval purposes, being always procured from fir by a similar process, is a compound of this substance and the turpentine of that tree. In the same manner the term of Pitch will here be applied to the solid material remaining after a gentle distillation of this tar; nautical pitch being a compound of common resin and this substance. The oil of turpentine, together with the oil of the wood, is separated in this latter case, as the oil of wood alone is in the former. I May here refer the reader to a paper in the second volume of the Geological Transactions, in which the subject has been examined in some detail with Other views.

The pitch, in a certain state of hardness, is the Bistre of artists, on which some remarks will also be found in the same paper.

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ammonia, often very minute, is also found, and this, as might be expected, is combined with the acid. When the process is irregularly conducted, a second process is carried on together with the first, and the new compounds become partially decomposed. Thus there is produced a gaseous substance, which is either hydrocarbonic gas, containing various proportions of carbon, or else a mixture of that gas and carbonic oxide. At times, according to the mode in which the process has been conducted, no essential oil is obtained, the whole of it being decomposed as fast as it is formed, and recompounded into the hydrocarbonic gas. But it may be procured by redistilling the tar, which thus becomes separated by a further decomposition into essential oil, pitch, and a fresh proportion of acetic acid, always holding, from its strong affinity to that substance, a portion of the inflammable compound in solution. The pitch, by continuing this process, at length loses its character, and becomes a brown or black friable mass, but still continues to give over volatile inflammable matter and acid, until it is finally reduced to the state of charcoal. Finally, it gives over hydrogen only, and acquires an extreme degree of hardness, as ordinary charcoal does in the same case, becoming also unsusceptible of further change. It may be remarked, that as the tar becomes pitchy, the essential oil, which was originally pale, and at first nearly colourless, becomes gradually darker, and that the volatile product of the solid pitch is not oil but tar, which is, however, capable of being decomposed in a similar manner by a repetition of the same treatment. It is obvious, that this is the consequence of the greater heat required in this case, to effect the separation of the volatile product.

In reconsidering the consequences of thus pushing the process to its extremity, it is apparent that the whole vegetable matter is or may, by further analysis of the acetic acid and the ammonia, be resolved into carbon, hydrogen, oxygen, and azote; that the proportions of these might, by a more laborious and accurate analysis, be discovered, and that they may be conjectured with sufficient accuracy for the purposes now in view, by attending to the several proportions in which these compounds are obtained.

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In stating these results, I have neglected to notice the fixed salts and earths which remain after the incineration of the charcoal, as they are not essential to the present inquiry. That I may not return to this subject, I will here remark, that, in pure peat, their nature and proportion will depend on the nature of the vegetables from which it has been generated, and that while potash, for example, may be found in the peat of submerged wood, silica will occur in that produced from gramineous vegetables. But as alluvial earths are frequently mixed with peat, the earthy residuum of this substance is generally in greater abundance, and often indeed amounts to a considerable quantity. Agriculturists may hence judge how little advantage is to be derived from peat-ashes as a manure, unless in those cases where a considerable quantity of burnt clay has been produced by the operation.

If imperfect peat be now subjected to the same operations, the same results are produced, but the proportions of the different products will be found to differ. In the first stages of the decomposition of plants, these differences are not very sensible, but they become gradually more so, as the peat subjected to examination is more perfect. When perfect peat, not yet entered on the stage of bituminization, is thus examined, it will be found that the proportion of acid is sensibly diminished, and that the more solid new compounds also bear a larger proportion to the more fluid. There are, in fact, produced more pitch and less essential oil, and consequently a larger quantity of charcoal, in proportion to the volatile products, is obtained, in the distillation of peat, than in that of recent, or undecomposed vegetables. Neglecting the azote in this case, it is therefore apparent, that the principal chemical change from vegetable matter to peat consists in a dissipation of part of the hydrogen and of the oxygen. But this is not the only change, as the solubility of peat in the various fluids formerly enumerated, proves that a new combination has also taken place among some of the elements. This soluble matter is analogous to that which is produced from the same elements by the action of fire, and is in both cases a compound of hydrogen and carbon principally. In the process, therefore, of converting vegetable matter into peat, the proportions of the original ingredients are not only

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changed, but the carbon and hydrogen enter to a certain degree into a new combination together with a portion of the oxygen, forming a compound analogous to that which results from the action of fire. The action of air and water on vegetable matter is therefore similar to that of fire, but is both much more tedious, and much less perfect, as the organization is never so thoroughly destroyed in the former as it is in the latter case. In the only exception in which the process of conversion seems equally perfect, it is limited to a small proportion of the total bulk of the material; and the pitchy matter already described as found in bogs, may be considered as a species of peat, of which the nature is completed to such a degree, as to differ but in very trifling circumstances from the pitch or bistre produced by the action of fire.

If the effects of fire in a less degree on vegetable matter be compared with those of water, the resemblance will still be apparent, as was already noticed in describing the natural solubility of peat in water. From the incipient action of fire, vegetable matter undergoes changes, by which the hydrocarbonaceous compound becomes partially soluble in water and in the other solvents of peat, while acetic acid is also generated; a portion of it being dissipated in vapour, and the remainder being soluble together with the brown matter; a circumstance very obvious in the familiar example formerly quoted, of the roasting of coffee. Some differences, it is true, are apparent, in continuing the comparison of these processes, but they do not appear difficult of explanation. Although the continued action of heat produces more acid and more of the soluble hydrocarbonaceous compound, these effects, in the operation of forming peat, diminish, or cease altogether, at a certain stage of the process. It must be considered, that the decomposition of the vegetable matter is in fact completed, as far as air and water can complete it, at an early stage, and that the remainder of the change into solid peat is rather of a mechanical than a chemical nature. The process is also far less perfect, and thus there is less acid produced; while that which is formed is carried away by the flowing of water as fast as it is generated, so as scarcely to be susceptible of examination; or is entangled in the solid peat, so as to be no longer exposed to the action of water.

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Even in this case, however, long maceration and heat still extract it. Another important remark on this subject was reserved to this place. Wood-tar is highly soluble in acetic acid in all its states, and is also invariably mixed with a portion of free acid, which may be separated from it imperfectly by water, and more completely by the alkaline solutions; after either of which operations it becomes more pitchy and tenacious. When this tar, therefore, appears to dissolve in water, it is in fact partly brought into solution by the acid. It is evident how this fact operates in comparing the solubility of peat with that of wood-tar; since the dissipation and final disappearance of the acid from the former, must, in a corresponding degree, diminish the solubility of the hydrocarbonaceous compound.

It does not appear that there are at present any further means of illustrating the chemical nature of pure peat, or the nature of the effects which are produced on the matter of vegetables by the action of air and water, as far as the generation of that substance in its simple state is concerned. At this point also, the analogy between the effects of fire and of water appear to cease, since the application of the former agent has, in, no direct experiment yet instituted for that purpose, produced the results which follow the ulterior action of the latter. These results are, the change from peat into lignite, through all its stages, down to the bituminization of the vegetable matter as it is found, nearly, if not absolutely, perfected in jet.

To illustrate this part of the subject, it is necessary to consider the distinction between the bituminous substances, and the hydrocarbonaceous compound which constitutes the essence of peat, and to inquire by what tests the two can be distinguished. A method of making this distinction is required for the purpose of ascertaining, if possible, the point at which the process of bituminization commences; and, by the result of the inquiry, it will be seen, that there is an essential distinction between peat and the bituminized woods, and that whatever confusion may appear between the two, follows from the admixture of both which occurs in certain states of the progress.

The comparison between bituminous wood and peat will be rendered most easy, and the actual distinctions made more

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striking, by selecting the simplest and extreme cases, namely, by placing in a parallel light the several bituminous substances and those products of vegetable matter already described, which are analogous to peat, and are produced by the action of fire.

It will be sufficient, in considering the bitumens, to enumerate the three most prominent varieties, namely, asphaltum, petroleum, and naphtha; the others may be considered as intermediate states or modifications of these. These substances are severally analogous to the pitch, the tar, and the essential oil of wood, and the changes which they undergo on the application of heat are in every respect similar. Thus, if petroleum be distilled, it is converted into naphtha and pitch, while the former, by a further application of heat, is decomposed, and furnishes hydrocarbonic gas. The asphaltum, by distillation, gives out petroleum, becoming gradually friable, until at length charcoal remains behind, as in the distillation of bistre. There is, moreover, in both cases, an uninterrupted transition from the essential oil to the charcoal. Thus naphtha, when of a brown colour, passes gradually into petroleum, as the oil of wood does into tar; and thus, during the prolonged distillation, both of petroleum and of tar, the oil gradually becomes thicker, while the matter in the retort, in both cases, becomes first tenacious, then brittle, and lastly friable, when a further continuance of the heat at length converts it into charcoal.

But, with this general analogy, there are important differences in the two sets of substances, united to some chemical characters common to both. There is, in the first place, a striking difference in the taste and smell, by which they can readily be distinguished; while, even the mixtures of the two, can be recognized by an union of the sensible qualities appertaining to both. These differences are most remarkable in the more fluid Varieties, but are sufficiently sensible in all. The action of alcohol, of ether, acetic acid, the alkalies, and the mineral acids, On both these classes of substances, in a great measure resemble each other. Certain differences will nevertheless be found; but it is unnecessary here to enter minutely into the subject, as the detail is not required for the purposes at present in view. The most remarkable distinction in their chemical relations is the fol-

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lowing. The essential oil of wood does not unite with naphtha, and, for this reason, while the vegetable tar and pitch are soluble in that oil, they are not soluble in the latter fluid. It produces indeed a slight effect at the boiling heat, but the dissolved portion is nearly all deposited on cooling. On the other hand, the bitumens unite readily among each other, and asphaltum is perfectly soluble in naphtha, while they, refuse to unite with the oil of wood. Naphtha offers, therefore, a sort of test by which the progress of bituminization can be discovered, if not with great nicety, yet sufficiently for the purposes required.

The nature of the change by which peat is converted into bituminized vegetable matter, or lignite, will also be best understood by examining the extreme case, or by comparing the chemical composition of the bitumens with that of the products obtained from vegetables by fire. This, in the present state of chemical knowledge, can only be done in the former, as it was in the latter case, by comparing the nature and proportions of the elements, as they are obtained, either simply or in new combinations, by the action of fire. A general account only of these is here required. It thus appears, in the first place, that hydrogen and carbon form the bulk of the bituminous substances, as they do of the vegetable products, since they are chiefly converted into hydrocarbonic gas. But they yield a far inferior proportion of acetic acid, while the ammonia again appears in larger proportion; whether from its being actually present in greater quantity, or rendered more sensible by the absence of the acid, has not been ascertained. For the present purposes, it is unnecessary to inquire more minutely into this part of the subject; and it is obvious, that the essential distinction between the bitumens and the vegetable products, is the different proportions in which oxygen enters into the two. In concluding these remarks on the chemical nature of all these substances, it is only necessary to observe, that an erroneous view has been entertained of the process by which the fluid varieties are converted into the solid, in consequence of mere exposure to air. As in the case of the conversion of oil of turpentine, first into common turpentine, and then into resin, this has been imagined to arise from the absorption of oxygen from the atmosphere. The result of their decomposition, however, shows that the real

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change is a change of proportion between the hydrogen and carbon, which constitute the chief part of their bulk; and that the action of the atmosphere consists in dissipating, in a manner not yet understood, a portion of the former substance, so as to increase the relative quantity of carbon. It is not, in fact, dissimilar in its effects to the action of fire.

In attempting to apply this knowledge for the purpose of distinguishing peat from lignite, or of ascertaining the progress of bituminization, it will be evident, that from the identity of the products obtained by destructive distillation, and the difficulty of ascertaining minute differences in their proportions, no satisfactory results can be derived from examining these proportions, in cases where the change is incipient, where, in effect, there is a mixture of peat and bituminized matter. In the last stages, the differences become more palpable, and in the case of jet, which is the most perfect of the lignites, they are abundantly sensible. But the test of naphtha may be applied to the examination, either of the oil procured by the distillation of peat, in its progress to bituminization, or to that obtained from the lignites. By means of this fluid, the mixed oil is separated; that which is produced from the peat or unchanged vegetable matter being rejected, while that which proceeds from the bituminized vegetable matter is dissolved. A tolerable conjecture can thus be formed of the degree in which the progress of bituminization has taken place; and, when considerable, it is generally also sensible in the smell of the oil, which resembles that produced by mixing naphtha with the oil of wood. The peculiar smell which some peat gives in combustion, arises from this cause, and indicates the commencement of bituminization.

In thus examining the various kinds of submerged vegetable matter to which we have access, the progress of bituminization may be traced, with little interruption, from peat, through submerged wood, brown coal, and surturbrand, down to jet, the most perfectly bituminized substance still retaining its organization, with which we are acquainted. That which I have chosen to consider as pure peat, gives no traces of bitumen; jet, on the contrary; gives very minute indications of the products obtained from unchanged vegetable matter. The state of the submerged papyri offers an interesting example of the progress

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of this change, since they contain a portion of bituminous matter soluble in naphtha, and it is undoubted that they owe this change to the action of water.

If peat which has undergone a commencement of bituminization be distilled, so as to separate the oil, it will be found that this is partially soluble in naphtha. Submerged wood, according to its condition, gives similar results, the specimens sometimes yielding ordinary tar, at others a mixture of tar and petroleum. In peat, the process stops thus at the outset; but, in submerged wood, it proceeds indefinitely. Thus Bovey (brown) coal gives out a mixture of these two products, in which the petroleum bears a conspicuous proportion. The surturbrand of Sussex produces similar results; but in both these cases, different proportions of petroleum seem to be procured from different specimens, according to the varying degree to which the process of bituminization has advanced. In jet, it is difficult to ascertain whether the products differ from those which are obtained from asphaltum or from coal; but in the few trials made with this view, the proportion of acid appeared to be greater in the former than in the latter case. Should this be the fact, the bituminization, even of jet, must be considered as incomplete, and it will probably be found to yield a small proportion of tar, or of the oil of wood.

From a consideration of all these circumstances, there seems no reason to doubt that peat is a substance intermediate between common vegetable matter and bituminous lignite, and that it is capable of undergoing the ultimate change, when all the requisite conditions are present. It equally follows, that as the action of water is capable of producing the incipient change, there is no reason to doubt that it may effect the ultimate one. This is indeed confirmed by the geological situation of brown coal, of surturbrand, and of jet, all of which occur in alluvial soils. In peat, time alone seems wanting to complete the process. The history of its formation, already given, proves that it is rarely of a distant origin; and, in those cases in which it shews marks of bituminization, it will probably be found that its antiquity is the highest. The bituminous lignites are always found buried under alluvial soil, sometimes of considerable depth; a circumstance generally sufficient to indicate a longer period of expo-

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sure to the action of the cause by which the change in the vegetable compound was induced. It is probable that the peat found under alluvial soils will be more highly bituminized than when it occurs in superficial beds; as in most cases it will, in these situations, be a more ancient deposit; but as yet no opportunities for making the requisite trials have been afforded.

It is impossible, by direct experiments, to prove that water, or the action of water and air combined, is capable of changing vegetable matter into bitumen, as the necessary element of time cannot be introduced into such experiments. It may, however, be inferred, with a considerable degree of probability, from analogical observations. It has already appeared that the essential distinction between the bitumen and the products of vegetable distillation, consisted in the predominance of oxygen in the latter class of substances. Apologizing for a little laxity in the use of the term, the process of bituminization must therefore be considered as a deoxydation of the vegetable matter. Now, when turpentine is converted into resin by the action of air, or of air and water, not only the proportions of hydrogen and carbon are changed, but it no longer yields on distillation the same quantity of acetic acid. A species of deoxydation has here therefore taken place, and analogous effects appear to result from the action of the same causes on the liquid bitumens, and on vegetable tar. It was also shewn that peat yielded less acid on distillation than unchanged wood, so that the action of water in this case also, has the power of deoxydating the vegetable matter, as well as of dissipating a portion of the hydrogen which it contains. It is not unreasonable to conclude, that it may, by a continuance of the same action, produce the ultimate change into bitumen. The changes which it is capable of effecting on the fibrous parts of animal matter, by converting them into adipocire, if they do not present a very exact analogy, at least indicate a power adequate to the production of effects more complicated and less to be anticipated.

It has been asserted that vegetable matter was converted into bitumen by the action of fire; and, in extending this theory to the formation of coal, some experiments have been adduced in support of its truth *. These consisted in heating vegetable

* Sir James Watt's experiments.

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matter in close vessels, under a species of pressure which was supposed to possess a power of modifying the results to this end. But the produce in this case is neither coal nor even asphaltum, nor is it in the slightest degree bituminized. It is, in fact, the pitch or bistre, produced in the ordinary process of the destructive distillation of wood; nor could even that solid substance be obtained, unless by some careless mode of conducting the experiment, so as to suffer the more volatile products to escape. It is also certain, that the effect which fire does not produce in the commencement of its action on vegetable matter, it does not produce at any stage; and that in no case is it capable, in our hands at least, of effecting the change from those substances into bitumen.

Thus far alone does it at present appear in our power to illustrate the nature of peat, and of the bituminated lignites with which it is so nearly allied. But it would be wrong to conclude this subject without entering, however slightly, on a much more conjectural field of inquiry, namely, the possible relation of peat to coal. This is a geological question of great importance, but on which, unfortunately, there is not much real information or solid reasoning to offer.

In examining the mechanical disposition of peat-mosses and of coal, a considerable resemblance presents itself. The former occur in strata more or less extensive, as the latter do; and in both, the strata vary in thickness in different parts, commonly disappearing in a thin edge. Strata of peat are also occasionally found, like coal strata, repeated above each other in the same place, and separated by beds of sand, clay, and marl; substances corresponding, in every respect but induration, to the sandstone, shale, and limestone of coal-fields. In both also, there are occasionally found remains of animals and vegetables, of which the organization is not destroyed; and this is most remarkable in the peat of estuaries, where these have been entangled in the earthy matters as they are in the shale of coal-fields. Where peat has been formed in lakes or extensive marshy cavities, it occupies distinct spots, separated from others of the same nature, as are those deposits of coal which have been called Independent Formations.

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The resemblance in composition between peat and coal is equally remarkable. In many coal strata, as in certain kinds of peat, no remains of organic materials can be detected. In others, remains of various plants are found, and these, as in peat, present botanical characters which indicate an aquatic origin. In some, fragments of wood abound, and the specimens of this nature bear a striking resemblance to the peat of forests. But this subject is trite, since it has been a frequent subject of remark.

The chemical differences between peat and coal are unquestionably great, as are the mechanical differences of texture and compactness; but it must be presupposed that the comparison is here to be drawn between a stratum of peat previously bituminized, and in which the change has been completed, even to a degree beyond that of jet. In this case, the chemical distinction ceases, and it remains only to account for the mechanical differences of compactness, and of a peculiar structure, analogous to that of shale, by which coal is distinguished from all the bitumens and bituminated lignites.

As it is here said that the chemical distinction between coal and lignite ceases at the period of complete bituminization, it is necessary to state more accurately what the real distinctions are between coal, asphaltum, and the bituminous lignites; all of these being substances which give results on distillation that correspond in a very perfect manner.

The composition of asphaltum, as it is commonly found in nature or produced by art, differs chiefly from that of coal in its containing a smaller proportion of carbon to the hydrogen, or in its yielding on distillation a greater quantity of petroleum, and a smaller quantity of residual charcoal. But even between these two substances there is sometime no such distinction, or it is very trifling; as very dry varieties of asphaltum will be found to contain as much charcoal as the fat or bituminous varieties of coal. The chief distinction, at all times, appears, as already suggested, rather of a mechanical nature; or, it is a difference of structure by which coal is chiefly distinguished from asphaltum. It is not difficult to conjecture how that difference has originated, as the coal has been apparently bituminized directly from the vegetable matter, while the asphaltum

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having once been liquid, its charcoal has originally been in a state of chemical combination with the hydrogen.

Between the chemical composition of coal and lignite, supposing the latter perfectly bituminized, there appears no difference; the only sensible one being that arising from the remains of organization visible in the latter. By a process of fusion, jet can in fact be converted into coal; or, at least, the slight difference which may remain, is that only which arises from the imperfect bituminization of the jet; a circumstance which does not affect this inquiry.

It remains, therefore, only to discover by what process beds of bituminized peat or lignite may have been so condensed as to acquire the peculiar compactness and mineral structure displayed by coal; as the chemical resemblance has been established, and as it has been shewn that the former are produced by the action of water on vegetable matter. Those who have considered the process of bituminization to have resulted from the action of fire, have also viewed the peculiar structure of coal as the consequence of fusion under pressure. The fallacy of the experiments by which the former circumstance was supposed to be established, has already been shewn. To determine the probability of the latter supposition by a more correct mode of experimenting, portions of jet were submitted to the action of fire, under pressure so regulated as to admit the escape of such volatile matter as might endanger the apparatus. Thus the organized structure of the jet was destroyed, and the mass was brought into fusion. On being cooled, it broke easily into irregular fragments, not unlike some varieties of coal. In all other respects, as might be expected, it bore an exact resemblance to that substance; and it is also worthy of remark, that in proportion to the greater or less facility with which the volatile matter escaped, the results bore an analogy, sometimes to dry, and at others to bituminous coal.

It does not, however, follow from these experiments, that the theory which they seem to support is the true one. There is, in the first place, a deficiency of evidence to prove, that the secondary strata in which coal is found have been exposed to the action of fire. In the next place, although the superincumbent weight of materials would undoubtedly give the degree of pressure required by this hypothesis, that pressure is not in it-

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self sufficient to answer all the required conditions. The porosity of the strata which surround coal, is generally such that they would not prevent the escape of the volatile matters which would be separated by the action of fire. The coal would therefore be charred in a great proportion of cases, while the surrounding materials would be impregnated with bitumen. That the former effect would take place, is proved by an occurrence by no means unfrequent, which may be considered as a direct experiment in illustration of this question, performed by nature on the great scale. This occurrence is the passage of trap-veins through beds of coal, the effect of which is to produce the consequences in question, notwithstanding the pressure of the superincumbent strata. The frequent occurrence of bituminous shale, or of bituminous sandstone in the vicinity of coal, admits of an easy explanation, without the necessity of recurring to the action of fire; by recollecting that the mixture of peat with clay and with sand, which is not unfrequent, would necessarily be converted into these compounds by the same causes which changed the pure peat into coal. Lastly, it is not possible to conceive, that had coal been produced by the fusion of peat or lignite, the forms of vegetables should have so often been preserved; since, in applying artificial heat, even under pressure, all marks of organization are obliterated; a consequence that might indeed be easily foreseen.

There appears no reason, therefore, to attribute the peculiar form and structure of coal to the action of fire, but the supposition is, on the contrary, attended by unsurmountable difficulties. On the other hand, there appears no difficulty in conceiving that these may have resulted from mere pressure. Jet and brown coal are unquestionably indebted to this for their compactness and form; and there is scarcely a greater difference in this respect between coal and compact peat, than there is between jet and common submerged wood. Nor is it very difficult to conceive that, under a state of minute division, aided possibly by the presence of water, coal might assume the peculiar mineral structure by which it is characterised. Under similar circumstances, shale appears to be produced from clay, and there is a striking analogy between the concretionary structure of this rock and that of coal.

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With respect to the comparative position of peat and of coal, as they relate to the earth's surface, they present no difficulty which is not easily removed by geological analogies. The difference depends solely on their relative antiquity, and it is no more difficult to imagine beds of peat deposited under the materials which have been converted into rocks, than to account for beds of conchiferous limestone, or for the accumulations of organic remains, both animal and vegetable, which are found deeply buried beneath the rocky strata of later origin.

ART. II. — Account of the Seven Icebergs of Spitzbergen *. By WILLIAM SCORESBY, Esq. jun. F. R. S. E. and M. W. S. &c.

ONE of the most interesting appearances to be found in Spitzbergen, is the Iceberg. This term, written Ysberg by the Dutch, signifies ice-mountain. I speak not here of the islands of ice which are borne to southern climates on the bosom of the ocean, but of those prodigious lodgments of ice which occur in the valleys adjoining the coast of Spitzbergen and other Polar countries, from which the floating icebergs seem to be derived. Where a chain of hills lies parallel to the line of the coast, and within a few miles distance of the sea-beach, having lateral ridges jutting towards the sea, at intervals of a league or two, we have a most favourable situation for the formation of icebergs. Such is precisely the nature of the situation a little to the northward of Charles' Island, where the conspicuous bodies of ice noticed by Martens, Phipps and others, and known by the name of the Seven Icebergs, occur. Each of these occupies a deep valley, open towards the sea, formed by hills of about 2000 feet elevation on the sides, and terminated in the interior by the chain of mountains, of perhaps 3000 to 3500 feet in height, which follows the line of the coast. They are exactly of the nature and appearance of glaciers; they commence at the margin of the sea, where they frequently constitute a considerable precipice, and extend along the valley, which

* Taken, with the author's permission, from his work on the Arctic Regions vol. i. p. 101: — ED.

VOL. II. No. 4. APRIL 1820. P


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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)

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