OF ARTS AND SCIENCES ! MAY 24, 1870. 231 



tinguished contemporary, Faraday ; but his influence on the progress of 

 science was not less marked or less important. Both of these eminent 

 men were for a long period of years best known to the English public 

 as teachers of Chemistry, but their investigations were chiefly limited 

 to physical problems ; yet, although both cultivated the border ground 

 between Chemistry and Physics, they followed wholly different lines 

 of research. While Faraday was so successfully developing the princi- 

 ples of electrical action, Graham with equal success was investigating 

 the laws of molecular motion. Each followed with wonderful constancy, 

 as well as skill, a single line of study from first to last, and to this con- 

 centration of power their great discoveries are largely due. 



One of the earliest and most important of Graham's investigations, 

 and the one which gave the direction to his subsequent course of study, 

 was that on the diffusion of gases. It had already been recognized 

 that impenetrability in its ordinary sense is not, as was formerly sup- 

 posed, a universal quality of matter. Dalton had not only recognized 

 that aeriform bodies exhibit a positive tendency to mix, or to penetrate 

 through each other, even in opposition to the force of gravity, but had 

 made this quality of gases the subject of experimental investigation. 

 He inferred, as the result of his inquiry, " that different gases afford 

 no resistance to each other; but that one gas spreads or expands into 

 the space occupied by another gas, as it would rush into a vacuum ; 

 at least, that the resistance which the particles of one gas offer to those 

 of another is of a very imperfect kind, to be compared to the resistance 

 which stones in the channel of a stream oppose to the flow of running 

 water." But although this theory of Dalton was essentially correct 

 and involved the whole truth, yet it was supported by no sufficient evi- 

 dence, and he failed to perceive the simple law which underlies this 

 whole class of phenomena. 



Graham, " on entering on this inquiry, found that gases diffuse into 

 the atmosphere with different degrees of ease and rapidity." This was 

 first observed by allowing each gas to diffuse from a bottle into the air 

 through a narrow tube in opposition to the solicitation of gravity. 

 Afterwards an observation of Doebereiner on the escape of hydrogen 

 gas by a fissure or crack in a glass receiver caused him to vary the 

 conditions of his experiments, and led to the invention of the well- 

 known " Diffusion Tube." In this simple apparatus a thin septum of 

 plaster of Paris is used to separate the diffusing gases, which, while it 

 arrests in a great measure all direct currents between the two media, 



