On.Calcarious and Gypjidus Earths. 41 



j[£l;HAVE obferved that two of the ingredients that enter into 

 the Gompofition of plants, to wit, the alkali and oil, were not 

 ufually found in Calcarious earths, the frefh fliells of fifli, and 

 a few others excepted, from which a portion of oil and a volatile 

 alkali may be extrafted. I prefumc, however, that an ample 

 fupply of thefe is neceffary to the vigorous health of plants; 

 and that Gypjious earths afford them by the following procefs. 



The affinity or attraction between alkalies and the acid of 

 Gypfuvi^ is ftronger than that which exifts between the flone 

 and its own acid. That is to fay, the acid will leave the ftone 

 or earth to unite with the alkali, as appeared by the experiments 

 I have mentioned, by which neutral falts were obtained by 

 leaching pulverized Calcarious (tones with the lees of afhes. 



When therefore an alkali comes in contraCl with pulverized 

 Gypfiiviy it will attract the acid, and combining with it and water, 

 form a neutral fait, while the Calcarious earth, deprived of its 

 acid, becomes cauftic. Let us then fuppofe Gypjum pulverized 

 and fpread thinly over the earth, it is evident that in this cafe 

 it expofes a large furface to the a6lion of the air; if this contains 

 a volatile or vegetable alkali, which I fliall by and by fhew that 

 it does, this alkali will feize upon the acid of the ftone, and 

 form a neutral fait. Salts I believe cannot chryftalize but by 

 the addition of water, which in their combination lofes its 

 fluidity and becomes a folid body ; the fluidity of water depends 

 upon its heat ; it muft therefore before it becomes folid, part 



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