ecomes less
and less active, until finally the molecule is moving so sluggishly
that when it collides with its fellow cohesion is able to hold it there.
Cohesion, then, has won the battle, and the gas has become a liquid.

Such, stated in terms of the mechanical theory of heat, is what is
brought to pass when a gas is liquefied in the laboratory of the
physicist. It remains only to note that different chemical substances
show the widest diversity as to the exact point of temperature at which
this balance of the expansive and cohesive tendencies is affected, but
that the point, under uniform conditions of pressure, is always the same
for the same substance. This diversity has to do pretty clearly with the
size of the individual molecules involved; but its exact explanation is
not yet forthcoming, and, except in a general way, the physicist
would not be able to predict the "critical temperature" of any new gas
presented to him. But once this has been determined by experiment, he
always knows just what to expect of any given substance. He knows, for
example, that in a mixture of gases hydrogen would still remain gaseous
after all the others had assumed the liquid state, and most of them the
solid state as well.

These mechanical conceptions well in mind, it is clear that what the
would-be liquefier of gases has all along sought to attain is merely
the insulation of the portion of matter with which he worked against the
access of heat-impulse from its environment. It is clear that were any
texture known which would permit a heat-impulse to pass through it in
one direction only, nothing more would be necessary than to place a
portion of gas in such a receptacle of this substance, so faced as to
permit egress but not entrance of the heat, and the gas thus enclosed,
were it hydrogen itself, would very soon become liquid and solid,
through spontaneous giving off of its energy, without any manipulation
whatever. Contrariwise, were the faces of the receptacle reversed, a
piece of iron placed within it would be made red-hot and melted though
the receptacle were kept packed in salt and ice and no heat applied
except such as came from this freezing mixture. One could cook a
beefsteak with a cake of ice had he but such a material as this with
which to make his stove. Not even Rumford or our modern Edward Atkinson
ever dreamed of such economy of fuel as that.

But, unfortunately, no such substance as this is known, nor, indeed, any