[108] January 1813.
[109] Edinburgh Review, vol. xxii. p. 270.
[111] These facts have been confirmed by agriculturists, who could not possibly have had any favourite theory to support. Dr. Fenwick tells us, (Essays on Calcareous Manures, p. 11. 1798,) that in the county of Durham, the farmers always distinguish between hot and mild limes. They never apply the former to exhausted lands, or to any soil that has been long under a course of tillage, unless it be very deep and rich. In peaty soils, and in new, sour, and wild lands, the hot limes, on the contrary, are preferred to the mild ones. Dr. Fenwick made some experiments to ascertain the cause of the differences between these varieties of lime; and though he failed to discover that by analysis which Mr. Tennant subsequently ascertained, he nevertheless arrived at a just conclusion by simple observation; and was led to believe, that "what farmers term hot limes, are such as re-absorb their fixed air more slowly, and therefore continue longer to exert the peculiar action of quicklime."
[112] When this substance was first introduced into America, which is nearly forty years since, it was imported from the quarries of Montmartre, and in such request was it, that a bushel of wheat was usually given for the same measure of gypsum: it is now, I believe, obtained from Nova Scotia; I have not heard that it has been found within the States. It may perhaps serve to convey some idea of the extent to which it has been applied, when I state, that Mr. Maclure assured me that not less than three hundred vessels are constantly employed in the traffic, and that in Philadelphia twenty merchants, at least, are engaged in supplying the demand for it. Its efficacy appears to be considerably increased by applying it in a minute state of division; and a want of attention to this circumstance may possibly have been one of the causes which have rendered its advantages less conspicuous in England. In America, three or four hundred mills, of a peculiar construction, have been erected in different parts for the purpose of grinding it.
[113] Gypsum is readily produced by the admixture of decomposing pyrites and calcareous matter: in proof of which the Mineralogist can produce specimens of oyster shells studded with crystals of selenite from Shotover; and alum from the aluminous shale at the Hurlet Mine near Glasgow.
Transcriber's note:
Minor spelling and punctuation inconsistencies been harmonized. Obvious printer errors have been repaired. Missing page numbers are page numbers that were not shown in the original text.
The formatting of the letters has been regularized.
Every effort to decipher the hand written letter by Sir Humphry Davy has been made, however, there are no guarantees that it is correct.