"It is only when powder is confined that it explodes," Tom
explained. "If it can burn in the open it's as harmless as water,
provided you don't burn too much at once. But put it in something
where the resulting gases accumulate and can't escape, and then--
why, you have an explosion--that's all."

"Yes--that's all," remarked Ned, grimly, as he nervously
watched the burning stick of powder. Tom let it flame for a few
seconds, and then calmly blew it out.

"You know what a little puff black gunpowder gives, if you burn
some openly on the ground," went on Tom; "don't you, Ned?"

"Sure, I've often done that."

"But put that same powder in a tight box, and set fire to it,
and you have a bang instead of a puff. It's the same way with
this powder, only it doesn't even puff, for it burns more slowly.

"An explosion, you see, is the sudden liberation at one time of
the gases which result when the powder is burned. If the gases
are given off gradually, and in the open, no harm is done. But
put a stick like this in, say, a steel box, all closed up, save a
hole for the fuse, and what do you have? An explosion. That's the
principle of all guns and cannon.

"But say, Ned, I'm getting to be a regular lecturer. I didn't
know I was running on so. Why didn't you stop me?"