the most
subtle of gases; and it has been made more and more apparent that gas
and liquid are, as Andrews long ago asserted, "only distant stages of
a long series of continuous physical changes." Of course, if the
temperature be lowered still further, the liquid becomes a solid; and
this change also has been effected in the case of some of the most
"permanent" gases, including air.

The degree of cold--that is, of absence of heat--thus produced is
enormous, relatively to anything of which we have experience in nature
here at the earth now, yet the molecules of solidified air, for
example, are not absolutely quiescent. In other words, they still have
a temperature, though so very low. But it is clearly conceivable that
a stage might be reached at which the molecules became absolutely
quiescent, as regards either translational or vibratory motion. Such a
heatless condition has been approached, but as yet not quite
attained, in laboratory experiments. It is called the absolute zero
of temperature, and is estimated to be equivalent to two hundred and
seventy-three degrees Centigrade below the freezing-point of water, or
ordinary zero.

A temperature (or absence of temperature) closely approximating this
is believed to obtain in the ethereal ocean of interplanetary and
interstellar space, which transmits, but is thought not to absorb,
radiant energy. We here on the earth's surface are protected from
exposure to this cold, which would deprive every organic thing of life
almost instantaneously, solely by the thin blanket of atmosphere with
which the globe is coated. It would seem as if this atmosphere,
exposed to such a temperature at its surface, must there be incessantly
liquefied, and thus fall back like rain to be dissolved into gas again
while it still is many miles above the earth's surface. This may be the
reason why its scurrying molecules have not long ago wandered off into
space and left the world without protection.

But whether or not such liquefaction of the air now occurs in our outer
atmosphere, there can be no question as to what must occur in its entire
depth were we permanently shut off from the heating influence of the
sun, as the astronomers threaten that we may be in a future age.
Each molecule, not alone of the atmosphere, but of the entire earth's
substance, is kept aquiver by the energy which it receives, or has
received, directly or indirectly, from the sun. Left to itself, each
molecule would wear