Mr. T. Graham on the Molecular Mobility of Gases. 423 



fusion into a gaseous atmosphere, through the same plate, are 

 due to the same inherent mobility of the gaseous molecule. 

 They are the exhibition of this movement in different circum- 

 stances. In interdiffusion we have two gases moved simulta- 

 neously through the passages in opposite directions, each gas 

 under the influence of its own inherent force ; while with gas on 

 one side of the plate and a vacuum on the other side, we have a 

 single gas moving in one direction only. The latter case may 

 be assimilated to the former if the vacuum be supposed to repre- 

 sent an infinitely light gas. It will not involve any error, there- 

 fore, to speak of both movements as gaseous diffusion, — the dif- 

 fusion of gas into gas (double diffusion) in one case, and the 

 diffusion of gas into a vacuum (single diffusion) in the other. 

 The inherent molecular mobility may also be justly spoken of as 

 the diffusibility or diffusive force of gases. 



The diffusive mobility of the gaseous molecule is a property 

 of matter fundamental in its nature, and the source of many 

 others. The rate of diffusibility of any gas has been said to be 

 regulated by its specific gravity, the velocity of diffusion having 

 been observed to vary inversely as the square root of the density 

 of the gas. This is true, but not in the sense of the diffusibility 

 being determined or caused by specific gravity. The physical 

 basis is the molecular mobility. The degree of motion which 

 the molecule possesses regulates the volume which the gas 

 assumes, and is obviously one, if not the only, determining- 

 cause of the peculiar specific gravity which the gas enjoys. If 

 it were possible to increase in a permanent manner the molecular 

 motion of a gas, its specific gravity would be altered, and it 

 would become a lighter gas. With the density is also associated 

 the equivalent weight of a gaseous element, according to the 

 doctrine of equal combining volumes. 



Diffusion of mixed gases into a vacuum, with partial separation — 



Atmolysis. 



Oxygen and Hydrogen. — A diffusiometer of the same construc- 

 tion as that described (fig. 3, p. 414), with a graphite plate of 

 1 millim. in thickness, was now employed. The upper surface 

 of the plate was swept by a current of the mixed gas proceeding 

 from a gas-holder, the excess of gas being allowed to escape into 

 the atmosphere, as usual, by an open exit -tube. The gas was 

 drawn through the graphite by elevating the diffusiometer con- 

 taining a column of mercury, from its well, so as to command a 

 partial vacuum in the upper part of the tube. Care is taken 

 that any gas, left in the upper part of the diffusiometer- tube 

 before the experiment begins, should be of the same composi- 

 tion as the gas to be allowed afterwards to enter, so that, on 



