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been informed by farmers in the neighbourhood of the 

 quarry, that they employ it advantageously in small 

 quantities, seldom more than 25 or 30 bushels to the 

 acre. And that they find it may be used with good 

 effect in larger quantities upon rich land. 



A minute chemical consideration of this question 

 will lead to its solution. 



Magnesia has a much weaker attraction for car- 

 bonic acid than lime, and will remain in the state of 

 caustic or calcined magnesia for many months, though 

 exposed to the air. And as long as any caustic lime 

 remains, the magnesia cannot be combined with car- 

 bonic acid, for lime instantly attracts carbonic acid 

 from magnesia. 



When a magnesian limestone is burnt, the mag- 

 nesia is deprived of carbonic acid much sooner than 

 the lime ; and if there is not much vegetable or ani- 

 mal matter in the soil to supply by its decomposition 

 carbonic acid, the magnesia will remain for a long 

 while in the caustic state ; and in this state acts as a 

 poison to certain vegetables. And that more magne- 

 sian lime may be used upon rich soils, seems to be 

 owing to the circumstance, that the decomposition of 

 the manure in them supplies carbonic acid. And 

 magnesia in its mild state, i. e. fully combined with 

 carbonic acid, seems to be always an useful constituent 

 of soils. I have thrown carbonate of magnesia (pro- 

 cured by boiling the solution of magnesia in super- 

 carbonate of potassa) upon grass, and upon growing 

 wheat and barley, so as to render the surface white ; 

 but the vegetation was not injured in the slightest de- 



