ts are sometimes observed a little after sunset in
the form of streamers or bands of light that shoot up into the sky,
sometimes to a great height. These are undoubtedly due to cloud
obstructions that partially shut off the sun's rays from a part of the
sky, but allow it to shine with greater brilliancy in the path of these
bands of light.

It will be seen from the foregoing that the sky in all of its phases is
a product of sunlight and the substances that float in the air,
including moisture, not only in the invisible state, but in all the
stages of condensation, as well as particles of floating dust.

CHAPTER XVIII.

LIQUID AIR.

Air, like water, assumes the liquid form at a certain temperature. Water
boils and vaporizes at 212 degrees Fahrenheit above zero, while liquid
air boils and vaporizes at 312 degrees below zero.

Heat and cold are practically relative terms, although scientists talk
about an "absolute zero" (the point of no heat), and Professor Dewar
fixes this point at 461 degrees Fahrenheit below zero. Others have
estimated that the force of the moon during its long night of half a
month, is reduced in temperature to six or seven hundred degrees below,
which is far lower than Professor Dewar's absolute zero. However this
may be, to an animal that is designed to live in a temperature of 70 or
80 degrees Fahrenheit, any temperature below zero would seem very cold.
If, however, we were adapted to a climate where the normal temperature
was 312 degrees Fahrenheit below zero, we should be severely burned if
we should sit down upon a cake of ice. Such a climate would be
impossible for animal existence, for the reason that there would be no
air to breathe, since it would all liquefy.

Liquid air is not a natural product. There is no place on our earth cold
enough to produce it. If the moon had an atmosphere (which it probably
has not) it would liquefy during the long lunar night, for heat radiates
very rapidly from a planet when the sun's rays are withdrawn from it.

As you have already surmised, liquid air is a product of intense cold.
Any method that will reduce the temperature of the air to 312 degrees
Fahrenheit below zero will liquefy it. Great pressure will not do this,
for we may compress air in a strong vessel until the pressure on every
square inch of the vessel is 12,000 pounds, or six tons, and still it
will not liquefy unless the temperature is brought down to the required
degree of cold