OF ARTS AND SCIENCES : MAY 24, 1870. 233 



have completely revolutionized the philosophy of Chemistry. In the 

 first place it has appeared that a host of chemical as well as of physi- 

 cal facts are co-ordinated by the assumption that all substances in the 

 state of gas have the same molecular volume, or, in other words, con- 

 tain the same number of molecules in a given space ; und in the second 

 place, it has become evident that the phenomena of heat are simply 

 the manifestations of molecular motion. According to this view, the 

 temperature of a body is the vis viva of its molecules ; and since all 

 molecules at a given temperature have the same vis viva, it follows 

 that the molecules must move with velocities which are inversely pro- 

 portional to the square roots of the molecular weights. Moreover, 

 since the molecular volumes are equal, and the molecular weights 

 therefore proportional to the densities of the aeriform bodies in which 

 the molecules are the active units, it also follows that the velocities of 

 the molecules in any two gases ai'e inversely proportional to the square 

 roots of their respective densities. Thus the simple numerical rela- 

 tions first observed in the phenomena of diffusion are the direct result 

 of molecular motion, and it is now seen that Graham's empirical law 

 is included under the fundamental laws of motion. Thus Graham's 

 investigation has become the basis of the new science of molecular 

 mechanics, and his measurements of the jates of diffusion prove to 

 be the measures of molecular velocities. 



From the study of diffusion Graham passed by a natural transi- 

 tion to the investigation of a class of phenomena which, although 

 closely allied to the first, as to the effects produced, differ wholly 

 in their essential nature. Here also he followed in the footsteps of 

 Dalton. This distinguished chemist had noticed that a bubble of air 

 separated by a film of water from an atmosphere of carbonic an- 

 hydride gradually expanded until it burst. In like manner a moist 

 bladder, half filled with air and tied, if suspended in an atmosphere 

 of the same material, becomes in time greatly distended by the in- 

 sinuation of this gas through its substance. This effect cannot be the 

 result of simple diffusion, for it is to be remembered that the thin- 

 nest film of water, or of any liquid, is absolutely impermeable to a gas 

 as such, and, moreover, only the carbonic anhydride passes through 

 the film, very little or none of the air escaping outward. The re- 

 sult depends, first, upon the solution of the carbonic anhydride by the 

 water on one surface of the film ; secondly, on the evaporation into the 

 air, from the other surface, of the gas thus absorbed. Similar ex- 



VOL. VIII. 30 



