ABSOLUTE ZERO
The volume of a gas varies inversely in accordance with the pressure to which it is subjected, and also directly according to the temperature. If we start with a gas at the freezing point (32 degrees F.) and reduce its temperature 1 degree (or to 31 degrees F.), we find that the volume of the gas is reduced 1/492.6 of its original volume, provided, of course, that we do not vary the pressure on it. In fact, for every reduction of 1 degree below the freezing point there is a reduction of 1/492.6 of its volume, and for every degree of increased temperature there is an increase of 1/492.6 of the volume. From this it is assumed that at 492.6 degrees below the freezing point, or 460.6 degrees below zero F., we will reach the absolute zero, or the point at which there is no more heat in the gas.
We have not yet succeeded in reaching the extreme of low temperature, although we have come very near it in laboratory experiments. Helium is liquefied at -448 degrees F., which is very near to the absolute zero. At the other end of the scale we have attained enormously high temperatures. The heat of the electric arc, for instance, is between 6,500 and 7,200 degrees F., and that is the highest degree of temperature that we have been able so far to attain.
Human life occupies a very limited zone in this range of temperatures. We must maintain our blood at a temperature of 98 degrees F. A variation of 8 degrees either way is fatal. By piling on heat insulators, such as fur clothing, to retain the heat of our bodies and keep it from flowing out too rapidly, we can maintain the blood temperature at 98 while the surrounding atmosphere may be 70 or 80 degrees below zero. There are internal fires within us that generate heat which radiates from the body, and by checking this radiation by suitable clothing we can maintain our blood at the normal temperature.
But what can we do when the surrounding temperature is higher than blood heat? The outside heat may be kept from flowing in by surrounding ourselves with heat-insulating clothing, but the internal heat then has no means of radiating away from our bodies; it accumulates, and we become overheated. However, Nature provides a cooling system in the perspiration which oozes from our pores, and as this evaporates it cools the skin and enables us to maintain our normal blood heat, although submerged in an atmosphere of a higher temperature. If the air is dry, the evaporation is more rapid and the cooling is greater than in a moist atmosphere. That is why a temperature of 105 degrees on our Western plains may be more endurable than a temperature of 95 degrees in the moist atmosphere of New York. The importance of keeping down the temperature of the blood is particularly appreciated by physicians, and for this reason the earliest attempts at artificial cooling were made by physicians.