88 SOURCE AND ASSIMILATION 



gypsum, it may be sufficient to remark that 100 

 Hess. Ibs. of burned gypsum fixes as much ammo- 

 nia in the soil as 6250 Ibs. of horses' urine * would 

 yield to it, even on the supposition that all the 

 nitrogen of the urea and hippuric acid were absorbed 

 by the plants without the smallest loss, in the form 

 of carbonate of ammonia. If we admit with Bous- 

 singault-f- that the nitrogen in grass amounts to 

 Too of its weight, then every pound of nitrogen 

 which we add increases the produce of the mea- 

 dow 100 Ibs., and this increased produce of 100 Ibs. 

 is effected by the aid of a little more than 4 Ibs. of 

 gypsum. 



Water is absolutely necessary to effect the de- 

 composition of the gypsum, on account of its diffi- 

 cult solubility, (1 part of gypsum requires 400 parts 

 of water for solution,) and also to assist in the ab- 

 sorption of the sulphate of ammonia by the plants : 

 hence it happens, that the influence of gypsum is 

 not observable on dry fields and meadows. 



The decomposition of gypsum by carbonate of 

 ammonia does not take place instantaneously ; on 

 the contrary, it proceeds very gradually, and this 



* The urine of the horse contains, according to Fourcroy and Vau- 

 quelin, in 1 000 parts, 



Urea 7 parts. 



Hippurate of soda . . 24 

 Salts and water . . 979 



1000 parts, 

 t Boussingault, Ann. de Ch. et de Phys. t. Ixiii. page 243. 



