CHAPTER VII 

 Magnesia and Silica 



Magnesia [MgO]. 



The question of magnesia has been much dis- 

 cussed of late years in consequence of certain trials 

 intended to demonstrate its efficacy as manure. 

 We think that the result of the magnesia in these 

 trials had not been satisfactorily isolated from that 

 of complementary agencies, and magnesia cannot 

 be brought into general use without danger, because 

 we should then have to pay more for kainite, owing 

 to the units of magnesia contained in it. In the 

 trials made the introduction of magnesia produced 

 a very notable increase of crops, including the 

 potato crop, and it also seemed to act favourably 

 on the size and quality of grain. Magnesia is chiefly 

 found in the salts of Stassfurt mixed with potash. 

 Raw kainite contains some chloride of magnesia 

 (MgCl2). Calcined kainite contains sulphate of 

 magnesia in considerable quantities. Chloride of 

 magnesia is caustic and, when spread on growing 

 crops, bums them. Moreover, in sacks it is de- 

 liquescent — that is to say it absorbs the moisture 

 from the air and makes kainite difficult to keep. 

 In dolomite magnesia is found in the form of 



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