-gases about which I shall have something more to say in
another chapter.

PRINCIPLES AND EXPERIMENTS

At first blush it seems a very marvellous thing, this liquefaction
of substances that under all ordinary conditions are gaseous. It is
certainly a little startling to have a cup of clear, water-like liquid
offered one, with the assurance that it is nothing but air; still more
so to have the same air presented in the form of a white "avalanche
snow." In a certain sense it is marvellous, because the mechanical
difficulties that have been overcome in reducing the air to these
unusual conditions are great. Yet, in another and broader view, there
is nothing more wonderful about liquid air than about liquid water, or
liquid mercury, or liquid iron. Long before air was actually liquefied,
it was perfectly understood by men of science that under certain
conditions it could be liquefied just as surely as water, mercury, iron,
and every other substance could be brought to a similar state. This
being known, and the principles involved understood, had there been
nothing more involved than the bare effort to realize these conditions
all the recent low-temperature work would have been mere scientific
child's-play, and liquid air would be but a toy of science. But in point
of fact there are many other things than this involved; new principles
were being searched for and found in the course of the application of
the old ones; new light was being thrown into many dark corners; new
fields of research, some of them as yet barely entered, were being
thrown open to the investigator; new applications of energy, of vast
importance not merely in pure science but in commercial life as well,
were being made available. That is why the low-temperature work must be
regarded as one of the most important scientific accomplishments of our
century.

At the very outset it was this work in large measure which gave the
final answer to the long-mooted question as to the nature of heat,
demonstrating the correctness of Count Rumford's view that heat is
only a condition not itself a substance. Since about the middle of the
century this view, known as the mechanical theory of heat, has been the
constant guide of the physicists in all their experiments, and any
one who would understand the low-temperature phenomena must keep this
conception of the nature of heat clearly and constantly in mind. To
understand the theory, one must think of all matter as com