my's boat at close-quarters. This really tremendous weapon
was an innocent-looking disc or circlet of gun-cotton, weighing not more
than eight ounces. Innocent it would, in truth, have been but for the
little detonator in its heart, without which it would only have burned,
not exploded. Attached to this disc was an instantaneous fuse of some
length, so that an operator could throw the disc into a passing boat,
and then fire the fuse, which would _instantly_ explode the disc.

All this was carefully explained by Firebrand to my astonished mother,
while the disc, for experimental purposes, was being placed in a cask
floating in the water. On the fuse being fired, this cask was blown
"into matchwood"--a wreck so complete that the most ignorant spectator
could not fail to understand what would have been the fate of a boat and
its crew in similar circumstances.

"How very awful!" said my mother. "Pray, Mr Firebrand, what _is_
gun-worsted--I mean cotton."

The young lieutenant smiled rather broadly as he explained, in a glib
and slightly sing-song tone, which savoured of the Woolwich Military
Academy, that, "gun-cotton is the name given to the explosive substance
produced by the action of nitric acid mixed with sulphuric acid, on
cotton fibre." He was going to add, "It contains carbon, hydrogen,
nitrogen, and oxygen, corresponding to--" when my mother stopped him.

"Dear me, Mr Firebrand, is a _popular_ explanation impossible?"

"Not impossible, madam, but rather difficult. Let me see. Gun-cotton
is a chemical compound of the elements which I have just named--a
_chemical compound_, you will observe, not a mechanical _mixture_, like
gunpowder. Hence it explodes more rapidly than the latter, and its
power is from three to six times greater."

My mother looked perplexed. "What is the difference," she asked,
"between a chemical compound and a mechanical mixture?"

Firebrand now in his turn looked perplexed. "Why, madam," he exclaimed,
in modulated desperation, "the ultimate molecules of a mixture are only
placed _beside_ each other, so that an atom of gunpowder may be
saltpetre, charcoal, or sulphur, dependent on its fellow-atoms for power
to act; whereas a chemical compound is such a perfect union of
substances, that each ultimate molecule is complete in its definite
proportions of the four elements, and therefore an _independent_ little
atom."

"Now, the next experiment," continued Firebrand, glad to have an