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Salt (see p. 117) Your correspondent "W. B. N"¹ must, I think, have seen salt from other salinas than those described by me;² probably (as I infer from his statement that the salt is brought into Buenos Ayres in ox-waggons), from the salinas north of S. Ventana. The salt from the Rio Negro, from the S. Chiquitas and from San Julian, instead of being an "amorphous mass," yielding "a soft powder," is coarsely crystallized, some of the cubes being even 3 or 4 inches square. Instead of being

¹ W.B.N. 1847. Native Patagonian salt. Gardeners' Chronicle no. 8 (20 February), p. 117.

² See Journal of researches 2d ed., pp. 65-7.

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mixed with much earth, the salt presents an expanse as white as newly fallen snow, which, viewed from a distance, as I well remember to my cost, might readily be mistaken for a lake. Your correspondent seems to think that by the term purity, I imply freedom from dirt, but in my work I explain that I mean, "the absence of those other saline bodies found in all sea-water,"— a remarkable fact, which I state after the careful analysis of Mr. T. Reeks¹ of the Museum of Econom. Geology. The salt consists entirely of chloride of sodium, with the exception of only 0.26 of sulphate of lime, and 0.22 of earthy matter. This fact having been ascertained, and the mass being well crystallised, it still appears to me that its lesser value for curing meat is probably owing to its purity, in the sense in which I have perhaps inappropriately used the term, that is, to the absence of those other saline substances found in sea-salt. I should not, however, have ventured on this opinion, had not Prof. Johnston² come to the conclusion "that those salts answer best for preserving cheese which contain most of the deliquescent chlorides."³ I must yet think that the experiment of adding some of the muriates of lime and magnesia to the salt from the Rio Negro, would be very well worth trial by the owners of the Saladeros near Buenos Ayres.—C. Darwin.

¹ Trenham Reeks (1823/4-1879), mineralogist. For Reeks' analysis of some of Darwin's mineral specimens from the Beagle voyage see Journal of researches 2d ed., p. 66 and Correspondence vol. 3, letter to T. Reeks, [before 8 February 1845], and letter from T. Reeks, 8 February 1845.

² James Finlay Weir Johnston (1796-1855), reader in chemistry and mineralogy at Durham University and the practical chemist of the Agricultural Chemical Association.

³ A paraphrase of comments printed in Report of the Agricultural Chemical Association. Gardeners' Chronicle and Agricultural Gazette no. 6 (8 February 1845): 93.