GYPSUM OR PLASTER. 217 



more or less broken, and compacted into a firm substance, 

 which falls on exposure to the air into a coarse white pow- 

 der. This substance is of considerable value. It may be 

 burned into a fair quality of lime, when it is of ^use for the 

 same purposes as stone lime. Or it may be spread on the 

 land after it has dried and become pulverized, as a substi- 

 tute for lime, with considerable benefit. 



SHELL LIME, procured by burning the shells of oysters 

 and other marine animals, has every useful property that 

 stone lime possesses; and as the lime is pure, with the ex- 

 ception of a small quantity of phosphoric acid which is 

 valuable this form of lime becomes a most important source 

 of supply to farmers near the sea coast or on the shores of 

 the large tidal rivers. 



LIMESTONE, ground into fine powder, has been offered to 

 farmers as a fertilizer of late years; but its almost insoluble 

 character renders it of questionable value, as compared with 

 lime, which can be procured at less cost because it needs no 

 grinding. Ground limestone is soluble only in water con- 

 taining considerable carbonic acid in solution, and then on- 

 ly to a small extent. Its value in special cases may be such 

 as to make its use desirable; but experimental tests are al- 

 ways required to discover its usefulness. No general rule 

 can be given in regard to it, excepting that its value is 

 wholly disproportionate to its cost, as compared with any 

 other form of lime. 



GYPSUM, is a compound of lime, sulphuric r.cid, and wa- 

 ter, in the proportion of o2l, 46 2, and 21 qarts of each, re- 

 spectively. Its remarkable action upon some crops, as 

 clover; peas; corn; cabbages; and turnips; has led to some 

 erroneous notions as to the causes of this action, and the er- 

 rors have been unfortunately fostered to some extent by 

 inexperienced writers upon agricultural topics. These 

 erroneous views are chiefly as follows. 



That gypsum gathers ammonia from the air and thus con- 

 tributes this useful substance to the plants. 



That it gathers moisture from the air and furnishes it to 



