There are other optical effects to be seen sometimes on the horizon somewhat resembling twilight. The "aurora borealis" (northern lights), which we describe in Vol. III., is seen in the northern skies at certain times, and has very much the appearance of twilight in some of its phases. It is constantly changing, however, and is easily distinguished by anyone who has observed both. These appearances are undoubtedly electrical. There is another phenomenon seen in the arctic regions that causes a band of white light to appear on the horizon called "ice blink," and it is caused by the reflections from the great icebergs that abound in that region.
Curious optical effects 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.