ays
a gas, regardless of pressure, is called the critical temperature, or
absolute boiling-point, of that substance. It does not follow, however,
that below this point the substance is necessarily a liquid. This is a
matter that will be determined by external conditions of pressure. Even
far below the critical temperature the molecules have an enormous degree
of activity, and tend to fly asunder, maintaining what appears to be
a gaseous, but what technically is called a vaporous, condition--the
distinction being that pressure alone suffices to reduce the vapor to
the liquid state. Thus water may change from the gaseous to the liquid
state at four hundred degrees above zero, but under conditions of
ordinary atmospheric pressure it does not do so until the temperature
is lowered three hundred degrees further. Below four hundred degrees,
however, it is technically a vapor, not a gas; but the sole difference,
it will be understood, is in the degree of molecular activity.

It thus appeared that the prevalence of water in a vaporous and liquid
rather than in a "permanently" gaseous condition here on the globe is a
mere incident of telluric evolution. Equally incidental is the fact that
the air we breathe is "permanently" gaseous and not liquid or solid,
as it might be were the earth's surface temperature to be lowered to a
degree which, in the larger view, may be regarded as trifling. Between
the atmospheric temperature in tropical and in arctic regions there is
often a variation of more than one hundred degrees; were the temperature
reduced another hundred, the point would be reached at which oxygen
gas becomes a vapor, and under increased pressure would be a liquid.
Thirty-seven degrees more would bring us to the critical temperature of
nitrogen.

Nor is this a mere theoretical assumption; it is a determination of
experimental science, quite independent of theory. The physicist in the
laboratory has produced artificial conditions of temperature enabling
him to change the state of the most persistent gases. Some fifty years
since, when the kinetic theory was in its infancy, Faraday liquefied
carbonic-acid gas, among others, and the experiments thus inaugurated
have been extended by numerous more recent investigators, notably
by Cailletet in Switzerland, by Pictet in France, and by Dr. Thomas.
Andrews and Professor James Dewar in England. In the course of these
experiments not only has air been liquefied, but hydrogen also,