Physical Properties of Matter in the Liquid and Gaseous States *



    Law of Gay-Lussac.—That the law of Gay-Lussac in the case of the so-called permanent gases, or in general terms of gases greatly above their critical points, holds good at least at ordinary pressures, within the limits of experimental error, is highly probable from the experiments of Regnault; but the results I have obtained with carbonic acid will show that this law, like that of Boyle, is true only in certain limiting conditions of gaseous matter, and that it wholly fails in others. It will be shown that not only does the coefficient of expansion change rapidly with the pressure, but that, the pressure or volume remaining constant, the coefficient changes with the temperature. The latter result was first obtained from a set of preliminary experiments, in which the expansion of carbonic acid under a pressure of seventeen atmospheres was observed at 4°, 20°, and 54°; and it has since been fully confirmed by a large number oi experiments made at different pressures and well-defined temperatures. These experiments were conducted by the two methods commonly known as the method of constant pressure and the method of constant volume. The two methods, except in the limiting conditions, do not give the same values for the coefficient of expansion; but they agree in this respect, that at high pressures the value of that coefficient changes with the temperature. While I have confined this statement to the actual results of experiment, I have no doubt that future observations will discover, in the case, at least, of such gases as carbonic acid, a similar but smaller change in the value of the co-efficient for heat at low pressures. The numerous experiments I have made on this subject will shortly be communicated in detail to the Society; and for the present I will only give the following results:—

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    Physical Properties of Matter in the Liquid and Gaseous States * . Nature 12, 321–322 (1875).

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