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THE following interesting letter has been forwarded to us by Mr. DARWIN. We have not been able to ascertain precisely to what plant the larger bodies belong, but we believe them to be the pollen grains of some Thistle or Centaurea.¹ They also bear a strong resemblance to the pollen grains of some Malvaceous plant,² but they are far larger than those of Malva sylvestris, the only species which could supply pollen in sufficient quantities to tinge the rain with a yellowish tint. Fir pollen is often carried by wind, and deposited by rain on leaves, and we have seen Oak pollen forming yellow spots on leaves after a shower.

A very slight shower, lasting hardly more than a minute, fell here this morning (July 2) about 10 o'clock.³ My wife gathering some flowers immediately afterwards noticed that the drops of water appeared yellowish, and that the white Roses were all spotted and stained. I did not hear of this circumstance till the evening; I then looked at several Roses and Syringas and found them much stained in spots. Between the petals of the double white Roses there were still drops of the dirty water: and this when put under the microscope showed numerous brown spherical bodies, 1/1000 of an inch in diameter, and covered with short, conical transparent spines. There were other smaller, smooth, colourless sacs about 4/7000 of an inch in diameter. I preserved a minute drop of the water beneath thin glass, cementing the edges, and next morning looked rather more carefully at it. I then observed that the water swarmed with elongated, moving atoms, only just visible with a quarter-inch object glass. Whether these inhabited the rain-drops, when they fell, I cannot of course say; but I suspect so, for the petals, now that they are nearly dry, seem stained with absolutely impalpable matter of the colour of rust of iron. This matter has chiefly collected, in the act of drying, on the edges of each spot. The Rev. M. J. Berkeley could tell us what the larger spherical bodies are which fell this day by myriads from the sky, carried up there, I presume, by some distant whirlwind.

We gathered a leaf spotted with yellow dusty patches a few days since in Mr. RUCKER'S⁴ garden at Wandsworth, but though there were grains of Fir pollen in the spots, and those of some other plant which we could not ascertain, together with a few spores of Fungi, the principal part of the matter consisted of slightly ferruginous apparently siliceous dust.

The ferruginous spots on the white petals of Philadelphus forwarded by Mr. DARWIN, consisted of coloured less distinctly siliceous particles, and multitudes of irregular bodies so minute as to present the Brownian molecular motion.

It is quite astonishing what a multitude of bodies are carried about by the wind in the form of dust. EHRENBERG⁵ some years since made us acquainted with the dust of the trade winds, but interesting matters may be found at home if we can in any way arrest the bodies which traverse our atmosphere. Flakes of snow bring down various things with them, and it is probable that few showers fall without leaving some sediment, though not so thick as in general to attract notice. An examination of such sediments or deposits with the microscope will soon materially modify our notions of spontaneous generation, and at the same time show a fertile source from which unexpected hybrid forms may arise. Indeed were not Fungi so much the creatures of peculiar atmospheric conditions, there would seem to be no limit to the diffusion of their species. M. J. B.⁶

¹ A large genus of herbaceous thistles and thistle-like flowering plants. See Correspondence vol. 11, p. 515.

² Mallows.

³ Darwin was writing from his home, Down House.

⁴ Sigismund Rucker (1809/10-1876), East and West India broker with premises in Great Tower Street, City of London who resided at West Hill, Wandsworth.

⁵ Ehrenberg 1844 and 1845.

⁶ Miles Joseph Berkeley.