AGRICULTURE WITH CHEMISTRY. 67 



SULPHAT OF LIME, OR GYPSUM. 



GYPSUM exists in great abundance, in many soils. It is 

 prodccedby the decomposition of alhiminous schyst, con- 

 taining a due proportion of calcareous matter; with 

 which the sulphuric acid will join, as it is formed 

 in preference to the earth of allum or clay. It is 

 likewise formed by the decomposition of pyrites, in 

 such soils as contain a sufficiency of calcareous mat- 

 ter for the sulphuric acid to combine with, in prefe- 

 rence to the earth of iron, the other constituent part of 

 pyrites : and it is found in immense quantities, constitut- 

 ing not only the soil, but the substratum, of some countries, 

 to a great depth. Gypsum is to be decomposed by alka- 

 line salts ; the sulphuric acid forming with them sulphat 

 of potash and sulphat of soda, according to the alkali 

 used. It is a salt very insoluble, requiring upwards of 

 five hundred times its weight of water to dissolve it : 

 hence supposing it equally deleterious to vegetation, as 

 allum hath been considered, which is soluble in only fif- 

 teen times its weight of water, it must prove less inju- 



i a rious, 



