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 ques- 
tion 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 com- 
bined with carbonic 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 
animal matter in the soil to supply by its decom- 
position 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 magnesian lime may be used upon rich soils, 
seems to be owing to the circumstance, that the 
decomposition of the manure in them supplies car- 
bonic 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 (procured 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 vege- 
tation was not injured in the slightest degree. And 
one of the most fertile parts of Cornwall, the Lizard, 
is a district in which the soil contains mild magne- 
sian earth. 
