compounds formed unite with the bases lime, soda, and potash, also
present, to form saltpetre.

Under all the conditions necessary, we see the permanent gases, oxygen
and nitrogen, leaving the atmosphere and changing from their gaseous to
a solid dry state, when they become chemically combined with potash, and
there are 53-46/100 parts of the gaseous matter and 46-54/100 parts of
the potash in 100 parts of the saltpetre by weight.

Having now found what saltpetre is and how it is formed, let us advance
to the consideration of it as a source of power.

Through the exertion of chemical attraction the gaseous elements of the
atmosphere have become solid in the saltpetre; and as we know the weight
of this part in a cubic inch of saltpetre, the volume of the gases
combined is easily ascertained to be about eight hundred times that of
the saltpetre. Hence, as every cubic inch of condensation represents
an atmosphere as large as the cubic inch of saltpetre formed, we may
roughly estimate that the condensing force arising from chemical
attraction in this case is 800 times 15 lbs., or 12,000 lbs.!

Strictly speaking, only about four-tenths of a cubic inch of potash
holds this enormous power in connection with it so as to form a cubic
inch of saltpetre, which we may handle and bruise, may melt and cool,
dissolve and crystallize, without explosion or change. It contains
conserved a force which represents the aggregate result of innumerable
minute actions, taking place among portions of matter which escape
our senses from their minuteness and excite our wonder by their
transformation. Closely similar are these actions to the agencies in
vegetation which build up the wood of the tree or the material of