1866.] Mr. Maxwell on the Dijnamical Theory of Gases, 169 



of time is equal to the rate of diminution of pressure of that gas as we pass 

 in the direction of the normal to the plane, multiplied bj a certain coeffi- 

 cient, called the coefficient of interdiffusion of these two gases. This co- 

 efficient must be determined experimentally for each pair of gases. It 

 varies directly as the square of the absolute temperature, and inversely as 

 the total pressure of the mixture. Its value for carbonic acid and air, as 

 deduced from experiments given by INIr. Graham in his paper on the 

 MobiHty of Gases, * is 



D=0-0235, 



the inch, the grain, and the second being units. Since, however, air is 

 itself a mixture, this result cannot be considered as final, and we have no 

 experiments from which the coefficient of interdiffusion of two pure gases 

 can be found. . 



3rd. AMien two gases are separated by a thin plate containing a small 

 hole, the rate at which the composition of the mixture varies in and near 

 the hole will depend on the thickness of the plate and the size of the hole. 

 As the thickness of the plate and the diameter of the hole are diminished, 

 the rate of variation will increase, and the effect of the mutual action of 

 the molecules of the gases in impeding each other's motion will diminish 

 relatively to the mo\dng force due to the variation of pressure. In the 

 Hmit, when the dimensions of the hole are indefinitely small, the velocity 

 of either gas will be the same as if the other gas were absent. Hence the 

 volumes diffused under equal pressures will be inversely as the square roots 

 of the specific gravities of the gases, as was first established by Grahamf ; 

 and the quantity of a gas which passes through a thin plug into another 

 gas will be nearly the same as that which passes into a vacuum in the same 

 time. 



(/3) By considering the variation of the total energy of motion of the 

 molecules, it is shown that, 



1st. In a mixture of two gases the mean energy of translation will be- 

 come the same for a molecule of either gas. From this follows the law of 

 Equivalent Volumes, discovered by Gay-Lussac from chemical considera- 

 tions ; namely, that equal volumes of two gases at equal pressures and tem- 

 peratures contain equal numbers of molecules. 



2nd. The law of cooling by expansion is determined. 



3rd. The specific heats at constant volume and at constant pressure are 

 determined and compared. This is done merely to determine the value of 

 a constant in the dynamical theory for the agreement between theory and 

 experiment with respect to the values of the two specific heats, and their 

 ratio is a consequence of the general theory of thermodynamics, and does 

 not depend on the mechanical theory which we adopt. 



4th. In quiet diffusion the heat produced by the interpenetration of the 



* Philosophical Transactions, 1863. 



t On the Law of the Diffusion of Gases," Transactions of the Royal Society of 

 Edinburgh, vol. xii. (1831). 



