66 FIRST PKINCIPLES OF AGRICULTURE. 



Gas-Lime. — The lime from gas-works is also fre- 

 quently used as manure ; in these works quicklime is 

 used for removing the impurities from the gas. Gas- 

 lime, therefore, varies considerably in composition, and 

 consists really of a mixture of slaked lime, or calcium 

 hydrate, and carbonate of lime, together with sulphides 

 and sulphites of lime. These last are injurious to plant 

 life, and gas-lime should be applied long before the crop 

 is planted, or at least exposed to the air some time 

 before its application ; the action of air converts the 

 poisonous substances in it into non-injurious products. 

 Gas-lime contains on an average forty per cent of cal- 

 cium oxide. 



Gypsum or Land Plaster. — Gypsum is a sulphate 

 of lime containing water in combination. Pure gypsum 

 contains thirty-two and one-half per cent of lime, forty- 

 six and one-half per cent of sulphuric acid, and twenty- 

 one per cent of water. 



Plaster of Paris is prepared from pure gypsum by 

 burning, which drives off the water it contains. Gyp- 

 sum, like other forms of lime, furnishes directly the 

 element calcium, and also exerts a favorable solvent 

 effect upon the soil. It was formerly used in large quan- 

 tities, particularly for clover; and it is believed that its 

 favorable effect was due, not so much to the direct addi- 

 tion of lime, as to its action upon insoluble potash com- 

 pounds in the soil, in setting free potash. Thus the 

 application of plaster caused an increase in crop because 

 of the potash made available. 



We have in the Eastern States two main sources of 

 gypsum, namely, Nova Scotia and Cayuga, N.Y. Nova 



