It has been seen that gunpowder was the explosive used to produce a vacuum
in Huyghens' engine, and that it was abandoned in favor of gas by Buren in
1823. The reason of departure is very obvious: a gunpowder explosion and a
gaseous explosion differ in very important practical points.

Gunpowder being a solid substance is capable of being packed into a very
small space; the gas evolved by its decomposition is so great in volume
that, even in the absence of any evolution of heat, a very high pressure
would result. One cubic inch of gunpowder confined in a space of one cubic
inch would cause a pressure by the gas it contains alone of 15,000 lb. per
square inch; if the heating effect be allowed for, pressures of four times
that amount, or 60,000 lb. per square inch, are easily accounted for.
These pressures are far too high for use in any engine, and the bare
possibility of getting such pressure by accident put gunpowder quite
outside the purpose of the engineer, quite apart from any question of
comparative cost. In a proper mixture of inflammable gas and air is found
an exceedingly safe explosive, perfectly manageable and quite incapable of
producing pressures in any sense dangerous to a properly constructed
engine.

The pressure produced by the explosion of any mixture of gas and air is
strictly determined and limited, whereas the pressure produced by the
explosion of gunpowder depends greatly upon the relation between the
volume of the gunpowder and the space in which it is confined.

Engines of the "Lenoir" type are the simplest in idea and construction; in
them a mixture of gas and air is made in the cylinder during the first
half of the piston stroke, air being taken from the atmosphere and drawn
into the cylinder by the forward movement of the piston. At the same time
gas entering by a number of holes, and streaming into the air to form an