MAGNESIA. 107 



mal and vegetable matter uniting itself with 

 the calcareous matter of the soil ; it is also found 

 to exist in the plaster of old buildings, which 

 have been long inhabited, especially it is abun- 

 dant in the plaster and cement of walls and 

 ceilings near privies, where, from the quantity 

 of nitrogen constantly exhaling, a considerable 

 portion is assimilated. 



Of its use as a manure nothing can be said ; 

 it is not known to exist in any state in plants, 

 and the few experiments I myself have insti- 

 tuted with a view to test its effects, have been 

 so unsatisfactory as to leave little hope of its 

 being beneficially employed hereafter. 



ON MAGNESIA. 



Magnesia^ the next earthy matter we have 

 to consider, is a constituent of almost all 

 plants, and exists abundantly in all parts of 

 the world, in various states of combination. 

 We have only to " recognise it here in the state 

 of a phosphate of magnesia, in which com- 

 bination it is found in the grain of all white 

 crops, and of which it forms a considerable 

 part. 



A native phosphate of magnesia is unknown, 

 the magnesia which we find therefore existing 

 as a phosphate in the corn and husks of all 

 grain, must be absorbed in the state of a car- 

 bonate, that is, dissolved in water by means of 

 carbonic acid gas, or it may be taken up in 

 combination with other acids, in which state 

 it is very soluble, and then being assimilated, 



