partly driven into pine wood, and then some pieces of dynamite placed on
the head of the nail, the latter may be struck hard blows with a wooden
mallet without exploding the dynamite _so long as the nail will continue
to enter the wood_.

[Footnote 1: The purest gun-cotton may be regarded as a _cellulose_,
in which three atoms of hydrogen are replaced by three molecules of
peroxide of nitrogen.]

Taking gunpowder as the unit, picrate of potash (picric acid and
potassium) has five times more force, gun-cotton seven and a half times,
and nitro-glycerine ten times more force. There are others still more
powerful, but less known and used, and some explosives are quite
uncontrollable and useless.

But the particular object of these remarks is to refer to articles of
merchandise non-explosive under general conditions, but so in particular
circumstances, as the two fire-extinguishers, water and salt, are
explosive under given conditions. The memorable fire which, in July,
1850, destroyed three hundred buildings in Philadelphia, upon Delaware
avenue, Water, Front, and Vine streets, was largely extended by
explosions of possibly concealed or unknown materials, the presence of
the generally recognized explosives being denied by the owners of the
properties.

"The germ of the first knowledge of an explosive was probably the
accidental discovery, ages ago, of the deflagrating property of the
natural saltpeter _when in contact with incandescent charcoal_."[1]
Although much manipulation is deemed necessary to form the close
mechanical mixture of the materials of gunpowder, it has never been