range of mutual attraction, two molecules may circle about
each other, as a comet circles about the sun, then rush apart again, as
the comet rushes from the sun.

It is obvious that the length of the mean free path of the molecules
of a gas may be increased indefinitely by decreasing the number of the
molecules themselves in a circumscribed space. It has been shown by
Professors Tait and Dewar that a vacuum may be produced artificially of
such a degree of rarefaction that the mean free path of the remaining
molecules is measurable in inches. The calculation is based on
experiments made with the radiometer of Professor Crookes, an instrument
which in itself is held to demonstrate the truth of the kinetic theory
of gases. Such an attenuated gas as this is considered by Professor
Crookes as constituting a fourth state of matter, which he terms
ultra-gaseous.

If, on the other hand, a gas is subjected to pressure, its molecules are
crowded closer together, and the length of their mean free path is thus
lessened. Ultimately, the pressure being sufficient, the molecules are
practically in continuous contact. Meantime the enormously increased
number of collisions has set the molecules more and more actively
vibrating, and the temperature of the gas has increased, as, indeed,
necessarily results in accordance with the law of the conservation
of energy. No amount of pressure, therefore, can suffice by itself to
reduce the gas to a liquid state. It is believed that even at the centre
of the sun, where the pressure is almost inconceivably great, all matter
is to be regarded as really gaseous, though the molecules must be so
packed together that the consistency is probably more like that of a
solid.

If, however, coincidently with the application of pressure, opportunity
be given for the excess of heat to be dissipated to a colder surrounding
medium, the molecules, giving off their excess of energy, become
relatively quiescent, and at a certain stage the gas becomes a liquid.
The exact point at which this transformation occurs, however, differs
enormously for different substances. In the case of water, for
example, it is a temperature more than four hundred degrees above zero,
centigrade; while for atmospheric air it is one hundred and ninety-four
degrees centigrade below zero, or more than a hundred and fifty degrees
below the point at which mercury freezes.

Be it high or low, the temperature above which any substance is alw