SCIENTIFIC AGRICULTURE. 



a valuable fertilizer, especially on lands deficient in clay and 



lime. 



GYPSUM. 



Gypsum is a compound of sulphuric acid, lime and water: 

 it is sometimes found in the form of a soft yellowish white rock, 

 with a texture resembling that of loaf sugar ; " but sometimes 

 [says Lyell,] it is entirely composed of lenticular crystals." It 

 is insoluble in acids, and does not effervesce, for the reason 

 that it is already combined with sulphuric acid, for which it 

 has a stronger affinity than for any other. A variety called 

 anhydrous gypsum sometimes occurs, which contains no water. 



Gypsum is nearly insoluble in pure water; when deprived 

 of its water by heat, it is called calcined gypsum, or " plaster 

 of paris," in this state, if mixed with water, it may be formed 

 into molds or casts, and it soon solidifies into a hard, white, 

 compact mass. When calcined gypsum is long exposed to the 

 air, it absorbs moisture, and is no longer fit for casts and stucco 

 work, until calcined afresh. Gypsum can only be fused by a 

 high degree of heat, it does not then part with its sulphuric 

 acid, but only loses its water. The origin of this rock is diffi- 

 cult to explain ; its found mostly among the new red sandstone, 

 but occurs also among other rocks. 



It is found in most countries in great abundance, and in 

 various forms, as gypseous spar, gypseous stalactites and sta- 

 lagmites, compact gypsum, &c., and in combination with clay 

 and lime. Gypsum cannot be formed artificially. Water con- 

 taining gypsum is called hard water. The decomposition of 

 gypsum can be easily effected by the alkaline carbonates: if 

 powdered gypsum be boiled in a solution of carbonate of 

 potash, a double decomposition, and also reunion, takes palce ; 

 the sulphuric acid of the gypsum unites with the potash, and 

 forms sulphate of potash, while the carbonic acid unites with 

 the lime of the gypsum and forms carbonate of lime. Gypsum 

 is one of the most valuable fertilizers known. 



