On the velocity of sound in mixtures of gases

In many investigations on the combustion of gases it is desirable to know the specific heat of amixture of gases of which the constituents may vary both as to their heat capacities, and as to the rate of change of these with temperature. For instance, in the determination of the ignition-points of gases and vapours with oxygen or air by the method of adiabatic compression we require to know the mean specific heats of the mixture between the initial and final temperatures ; and similarly in the more difficult problem of attempting to calculate the temperatures reached in the explosion of gases we require to know the heat absorbed by the mixed gaseous products. Again, if the explosion wave is propagated by a process analogous to that of a soundwave, it is important to be sure that the motion of a sound-wave, in a mixture of gases of different densities, can be calculated. Now very few experiments have been published on the velocity of sound in mixtures of gases—except in the case of nitrogen peroxide, where the dissociating gas mixture is in a state of mobile equilibrium easily altered by slight changes of temperature and pressure. It appeared, therefore, of interest to measure the velocity of sound in a few mixtures of simple gases, and to compare the results with those derived from Laplace’s formula in which the ratio of the specific heats is calculated from the expression :-- γ = Cp a + C'p b /Cv a + C'v b , where Cp and Cv are the specific heats of the gas A, at constant pressure and constant volume, and C'p and C'v are similarly the specific heats of the gas B, anda andb are the volumes of A and B respectively.