819 MANURE — &i?SUM. 



in the azote of the atmosphere, or in both simultaneously ; b it the 

 opinion which should maintain that the ammonia derived from the 

 organic constituents of the soil, passes into the state of nitric acid 

 before penetrating the tissues of plants, would find support nearly in 

 the same facts which I have quoted as favoring the former view. 

 We have seen, moreover, in our general considerations on nitrifica- 

 tion, with what facility the azote of ammonia undergoes acidification 

 in certain circumstances, a fact from which an argument of much 

 potency for the nitric acid theory naturally flows. I shall here add 

 an observation to which I have, up to this time perhaps, attached 

 too little importance. When M. Rivero and I examined the hig: ly 

 irritating and poisonous milky sap of the hura crepitans, we had oc 

 casion to leave a considerable quantity of the water derived from 

 the sap, after separating the caseum, to itself; by the spontaneous 

 evaporation of this vi'ater, we collected really a considerable quanti- 

 ty of nitrate of potash. Since this time I have had occasion to 

 note the same salt in the sap of several trees of the tropics. In the 

 leaves and fruit, however, I have never found more than very minute 

 qu^l.cities. 



Gypsum, sulphate of lime, or pla^ier of Parts, is a compcjnd of 

 41.5 lime with 58.5 sulpnuric acid; gypsum generally contains a 

 quantity of constitutional water, in which case it consists of 79.2 

 sulphate of lime, and 20.8 water = 100. This hydrate of sulphate 

 of lime is one of the abundant minerals on the surface of the earth ; 

 it is met with in the crystalline state, and in granular and fibrous 

 masses in the strata of most recent formation. It has no sensible 

 taste, but is slightly soluble in water, this fluid dissolving ^ of its 

 weight of the salt. Exposed for some time to a white heat, it loses 

 its water of constitution, and passes into the state in which when 

 ground it is known under the name of plaster of Paris. 



Gypsum is one of the most commonly employed of the mineral 

 manures. Its virtues appear not to have been unknown to the an- 

 cients : but until lately its employment was limited to a few circum- 

 scribed districts. It was only about the middle of the eighteenth 

 century that the protestant pastor, Mayer, took up the study of gyp- 

 sum in the principality of Hohenlohe, proceeding upon certain in- 

 formation which he had obtained from Hehlen of Hanover, in the 

 neighborhood of which, it was employed as an improver. 



By extending a knowledge of the virtues of gypsum, both by his 

 example ard his writings, Mayer did great service to agriculture. 

 Experiments were soon instituted in all quarters. Tschiffeli in 

 Switzerland, Schubart in Germany, and Franklin in America, wrote 

 on its effects, or practically demonstrnted them to the satisfaction of 

 all. But it appears to be the fate c " all useful discoveries, of all 

 happy applications of principles, to be opposed at first, and only to be 

 admitted after having been vainly disputed. The use of gypsum 

 soon aroused formidable opposition ; and there is a curious episodf 

 in the history of the paper war that was long carried on upon hi 

 •ubject, which I think worth noting. Among the most strenuous 

 •oemies of the use of gypsum, were the proprietors of the saJt-paiUb 



