C. GENERAL PHYSICS. 171 



exposed by Sir William Thomson. On either theory, it would 

 seem as it* the universe needed the application of some ex- 

 terior power in order to maintain the existing state of things. 



12.4,1873,279. 



THE DYNAMICAL THEORY OF GASES. 



The followins: remarks from Stefan's memoir on the con- 

 d action of heat by gases are worthy of being reproduced, and 

 will show the rapid progress recently made of the dynamical 

 theory of gases. 



Stefan says, "The dynamical theory of the constitution of 

 gases explains the pressure as the effect of the impulse of 

 rapidly moving particles." From the known pressure and 

 densitv, Clausius has deduced one of the constants that char- 

 acterize the nature of every gas, viz., the velocity of the pro- 

 gressive motion of the gaseous molecules. By the same 

 theory. Maxwell has explained the internal fiiction of gases 

 as the effect of the exchano-e of velocities between the mole- 

 cules moving at different rates in different strata. From 

 the observations of Graham on endosmose, from the pendu- 

 lum experiments of O. L. Meyer, and from the value of the 

 internal friction as deduced by Maxwell, the latter has ar- 

 rived at a second constant peculiar to the nature of the gas, 

 viz., the number of impacts that take place between the 

 molecules of the gas in a unit of time, and the mean length 

 of the path pursued by a molecule between two successive 

 impacts. From the same theory, however, it foUoAvs that, by 

 means of the two constants thus found, there may also be 

 determined the velocity of diffusion and the coefficient of con- 

 duction of heat. A series of determinations of the velocity 

 of diffusion has been made by Yon Loschmidt for various 

 combinations of gases, and Stefan finds that, by the use of 

 tiie above-mentioned previously known constants peculiar to 

 each gas, we could have predicted the results of Loschmidt's 

 observations. On the other hand, the conductivity for heat 

 has been already computed in advance by Maxwell and 

 Clausius from theoretical considerations. Therefore, for the 

 more complete establishment of the dynamical theory of 

 gases, it is of the greatest importance to have an experimental 

 determination of this same quantity. After giving an account 

 of the numerous difficulties that had hitherto prevented the 



