Page:Popular Science Monthly Volume 59.djvu/600

590 be answered: Does a molecule of helium or of argon resemble a molecule of oxygen in consisting of two atoms; or does it consist of one atom or of more than two?

It is believed that when heat is put into a gas, it is expended in causing the molecules to move. This motion may in some cases be of two kinds; the molecules may be urged through space, each molecule traveling in a straight path, until a collision takes place with another molecule, when it changes its rate and direction of motion; such motion i? termed 'translational motion.' On the other hand, if the molecules are themselves complex, that is, if they consist of groups of atoms, any energy imparted to the gas in the form of heat will produce, not merely the translational motion, but will also cause the atoms to move relatively to each other within the molecule. It is only the translational motion which is manifested as pressure, for it is only by their motion through space that the molecules can bombard the sides of the vessel which contains them, and so exert pressure on the walls. Hence it will require a less amount of heat to raise pressure in a gas with simple molecules, than in one of which the molecules are complex, for in the former case no heat is used in causing internal motion. Now, to measure such quantities of heat is by no means easy, although it has been successfully accomplished in some instances. To avoid this measurement, a device is adopted which produces equally satisfactory results. It consists in comparing the amounts of heat required to raise the temperature of a gas, first, when it is not allowed to expand, and when all the heat is used in producing molecular motion of the kind referred to; and second, when it is allowed to expand, and consequently when it could be made to do work; for example, to drive a small air-engine. In the latter case, a greater amount of heat is required to rise the temperature of the gas; an amount equivalent to the work which the gas does on expanding. This quantity, however, which is equivalent to mechanical work, is the same for all gases, provided equal numbers of molecules (or equal volumes) be heated through the same number of degrees of temperature. And this renders it possible to calculate the amount of heat required to raise the temperature of a gas, even without a direct measurement. An example will serve to render this somewhat difficult conception clear. For mercury-gas, for argon and for helium, and indeed for all gases, nitrogen, oxygen and their mixture, air, if a volume which contains the molecular weight of the gas taken in grams be raised through one degree of temperature, allowing the gas to expand, and so to do work, the amount of heat equivalent to this work is sufficient to raise the temperature of two grams of water through one degree. This is termed in the language of heat-measurement 2 calories. Now, the total heat required to raise the temperature of 40 grams of argon, for example (and it must be remembered that 40 is