On the Mforbenl Powers of difereni Earths. 197 



degrees offeree, modified alfo by temperature, ivhich affeSis the meafure of all combinations. The 

 farts t!ius obtained, though not of a brilliant nature, appear highly inflruQive, and if 

 profecuted with attention, might throw feme light on the obfcure theory of vegetation, 

 and fuggeft ufeful improvements in agriculture and rural (Economy. I give this early 

 notice of the little which I have done, as it feems to diverge from that line of inquiry 

 which I had prefcribed to myfelf, and as I fliall not at leafl for the prefent, purfue it 

 farther. But I eaineftly invite chemifts and naturalids to refume a fubjedl which promifes 

 fuch an eafy and abundant harveft. 



My pr6cedure was this: — The earths, or the ftony matters grofsly pounded, were dried 

 thoroughly before a good fire, and immediately introduced into phials, which were (lopped 

 up and fet afide to cool. The firft objedl was to afcertain, whether the degree of heat to 

 which they were fubjefted in drying, would aiFefl: their abforbent quality. I foon per- 

 ceived, however, that the refults were quite regular and uniform. In fa£l, the powder 

 is dried in this cafe not by the aftion of the hot air on its furface, but by the heat pene- 

 trating the mafs, and communicating to the aqueous particles the difpofition to aflume the 

 vaporous ftate, with the correfponding elaftlcity of fteam, which beyond the boiling point 

 increafes with moft rapid progreflion, and which enormous diftenfive power foon over- 

 comes the obftinate adhefion to the earthy bafis. Any heat almoft between the 100"" and 

 300"' degree of the centigrade fcale, will bring the fubftances into the proper condition. 

 This deficcated powder is ftrewed on the bottom of a faucer holding an hygrometer, over 

 which a fmall receiver is inverted. In a few minutes the inftrument marks the higheft 

 degree of drynefs produced. Here are a few of the principal refults Indicated by this 

 mode, at the temperature of 16° centigrade. 



Free ftone 37' 



Fine fea fand 40 - 



Marble ...*... 47 



Common clay . . i . . . 68 

 Sea fand, lately cultivated ... 72 



Sandy fchift ..... 75 



Bog earth 77 



Rotten whinftone ..... 78 

 Garden mould ..... 80 



It is remarkable that marble and quicklime produce exadlly the fame effeft, and that in 

 general no fenfibie difference is perceived between the pure earths and their carbonates. 

 The great abforbent power of ihe argillaceous, compared with the filicious bodies, alfo de- 

 ferves particular notice. But the cultivated foils poffefs that power in the moft confpicu- 

 ous degree. Garden mould ftands at the top of the fcale, and even fea fand, which caufes 

 a drynefs of 40° only is, by a few years' tillage, rendered capable of producing that of 72°. 

 Nor is it the operation of manure, for the fingle effe£l of this material is inferior to that of 

 moft of the earths. It feems highly probable, that the fertility of foils depends chiefly on 



their 



