The law connecting the volume of a gas with the temperature was discovered by Gay-Lussac, and independently by Dalton; but it is generally attributed to the former chemist. It is :—Provided pressure be kept constant, the volume of a gas, measured at 0° C, increases by 1/273 for each rise of 1°. Or 1 volume of gas at o° will become 1.00367 volume at 1°; 1.0367 volume at io° ; 1.367 volume at ioo°, and so on. Generally stated, if t stand for a temperature, 1 volume of gas will become 1 +0.00367/ when heated from o° to that temperature.

A third law may be deduced from these two; it is, that if the volume of a gas be kept constant, the pressure of a gas will increase 1/273 of its initial value at 0° for each rise of 1°. This is evident from the following consideration:—Suppose that 1 volume of a gas is heated from 0° to 1°; the volume will increase to 1.00367 volume. To reduce the volume again to its initial value, 1, the pressure must be raised by 0.00367 of its original amount. If the initial pressure corresponded to that of 76 centimeters of mercury, it would have to be increased to 76+(76 x 0.00367) centimeters, or to 76.279 centimeters in order that the gas should resume its original volume of I. The same consideration will hold if the gas is cooled instead of being heated; but of course in that case the pressure will be reduced, instead of being raised. It follows from this, that if the temperature could be reduced to 273 below o° C, the gas would exert no pressure. This temperature, — 273°, is termed " absolute zero." As a matter of fact, so low a temperature has never been reached; and, moreover, it is certain that all gases would change to liquids before that temperature was attained. But it serves as the starting-point for what is termed the " absolute scale of temperature." Gay-Lussac's law may therefore be stated thus:— The volume of a gas at constant pressure increases as the absolute temperature ; and its corollary, thus:— The pressure of a gas at constant volume increases as the absolute temperature. For o° C. corresponds with 273⁰ on the absolute scale; and 273 volumes of gas will become 274, if the temperature is raised from 273⁰ absolute to 274° absolute. Similarly, the pressure of a gas will increase in the proportion 273 : 274 if the absolute temperature is increased from 273⁰ to 274⁰.