46 
MANURES. 
saltpetre, by changing its potash for soda. One step 
more. I want you to understand by saltpetre, not one 
salt, but, in farming, a class of salts; that is, a num¬ 
ber, having the same acid, which may be combined 
with several different bases which all act one way. 
Saltpetre being a salt, of course must be composed of 
an acid and a base. The acid is always aqua-fortis, or 
nitric acid. The base may be potash, or soda, or lime, 
or ammonia. These all may be called saltpetre. In 
forming saltpetre, it is generally that variety which 
contains lime and aqua-fortis which is procured. So 
far as we understand the action of salts, and this has 
been fully explained, the action of the varieties of 
saltpetre is the same ; and were it not for the peculiar 
nature of the aqua-fortis, or acid of saltpetre, the ex¬ 
planation of the action of this salt might be referred 
to the general laws above set forth. But the acid of 
saltpetre is composed of volatile ingredients. It is 
nothing more nor less than a compound of the com¬ 
mon air we breathe. Surprising as it may seem, 
reader, yet it is not the less true, the common air is a 
mixture of oxygen and nitrogen. What a bland and 
harmless, yea, what a healthful blessing is air, not 
only to us, but to plants ! It is a mere mixture, not a 
chemical compound, a mere mixture. In every hun¬ 
dred parts, eighty of nitrogen, twenty of oxygen. Yet 
if you compel, as natural operations are continually 
compelling the air to unite chemically, so that four¬ 
teen par of nitrogen shall unite to forty parts of 
oxyge you will form aqua-fortis. Now, I do not 
mean .o trouble your head further with the chemistry 
of saltpetre, than merely to say, that having thus 
shown you the composition and origin of the acid of 
all kinds of saltpetre, you will readily see, that a sub¬ 
stance which affords such an abundance of nitrogen 
cannot but be beneficial to plants. This nitrogen 
may, and probably does, form some portion of ammo¬ 
nia in the soil. It may enter as nitrogen into the 
plants, dissolved in water, as a very weak aqua-fortis 
