Page:International Library of Technology, Volume 93.djvu/45

 pressure has, of course, been unchanged. If the air is warmed 10°, or from 32° F. to 42° F., the upward movement of the piston will be ten times as great; and for an increase of temperature of 100°, it will be one hundred times as great. The expansion of the air for an increase of 1° in temperature at any point on the scale, as, for, example, from 200° F. to 201° F., is practically the same as from 32° F. to 33° F. Similarly, for temperatures below 32° F., the contraction is in the same proportion. The amount of this change of volume, whether by expansion or contraction, is $1⁄492$ of the volume of the air at 32° F., for each degree of change in temperature. The sfraction $1⁄492$ is therefore called the coefficient of expansion of air. The result of this experiment is embodied in the law of Gay-lussac, as follows:

If the pressure of a gas remains constant, the increase of volume per degree rise of temperature is $1⁄492$ of the volume at 32° F.

From this, it is apparent that when a gas is heated or cooled at constant pressure, the volumes will be directly proportional to the absolute temperatures.

Gay-Lussac's law may be supplemented by two others, as follows:

1. ''All gases have practically the same coefficient of expansion, at constant pressure, as air; that is, $1⁄492$. ''

2. ''This coefficient is the same, whatever the pressure to which the gas may be subjected. ''

35. The laws just mentioned are not absolutely exact, since they express the expansion of gases approximately only. However, unless a gas is compressed very highly or is cooled to a point near that at which it liquefies, these laws are quite nearly true. The fact that all gases have practically the same coefficient of expansion as air is important, inasmuch as it simplifies the work of investigating the changes in volume of a gas under changes of temperature and pressure.

For example, if a quantity of oxygen at 32° F., under atmospheric pressure, occupies a volume of 1 cubic foot, and the temperature is raised to 2,000° F., the pressure remaining