(97.)

Another law, common to all elastic fluids, and of equal importance with the former, was discovered by Mariotte. By this law it appears that every gas or vapour, so long as its temperature is unchanged, will have a pressure directly proportional to its density. If therefore, while we compress steam into half its volume, we could preserve its temperature unaltered, we should increase its pressure in a two-fold proportion; but if the process of compression should cause its temperature to increase, [Pg172] then its increase of pressure will be greater than its increase of density, since it will be due conjointly to the increase of density and to the increase of temperature. In this case the increased pressure may be deduced from the combined application of the two laws just explained; that of Mariotte will determine that increase of pressure which is due to the increase of density, and that of Dalton and Gay Lussac will determine the further increase of pressure which will be due to the increase of temperature. The full investigation of these effects, and the formulæ expressing them, will be found in the Appendix to this volume.