200 PROFESSOR THOMAS GRAHAM's SCIENTIFIC WORK. 



point, he replaced tlie Assured jar by an instrument admitting of 

 much greater experimental indecision. For the jar itself he substituted 

 a piece of glass tube about half au inch in diameter, and from eight to 

 fourteen inches long, and for the fissure in the jar he substituted a 

 plate of stucco serving to close one end of the tube. Operating with a 

 diffusion-tube of this kind standing in ajar of water, it was found, as 

 in Dalton's experiments, that the two gases, say external air and internal 

 hydrogen, exhibited a powerful tendency to intermix or change places 

 with each other ; but more than this, it was found that the air did not 

 exchange with its own volume of hydrogen, but instead with 3.8 times 

 its volume. Using the word diifusion-volume to express the bulks of 

 different gases exchanging thus with one another by the process of 

 diffusion, the diffusion-volume of hydrogen would be 3.8, that of air being 

 taken as 1. Similarly, it was ascertained that every gas has a diffusion- 

 volume which is peculiar to itself, and is indeed inversely as the square 

 root of its specific gravity ; and since the unequal diffusion volumes of 

 different gases are consequences of their unequal diffusion velocities, it 

 follows that the relative velocities at which different gases diffuse into 

 one another, by virtue of their own inherent mobility, are identical with 

 those at which they effuse under pressure into a vacuum— a result quite 

 in accordance with, and indeed deducible from, Dalton's aphorism. But 

 although the relative rates of effusion and diffusion are alike, it is 

 important, wrote Mr. Graham, in the later paper already quoted from, 

 " to observe that the phenomena of effusion and diffusion are distinct 

 and essentially different in their nature. The effusion movement affects 

 masses of gas, the diffusion movement affects molecules ; and a gas is 

 usually carried by the former kind of impulse with a velocity many 

 thousand times as great as is demonstrated by the latter."* 



Thus the result arrived at by Mr. Graham, in his original paper, was 

 the enunciation of the now well-recognized law of the diffusion of gases ; 

 but some thirty years afterward, he again subjected the phenomena of 

 gas-diffusion to an elaborate experimentalinvestigation— going over the 

 old and penetrating into new ground with an activity by no means im- 

 paired, and with intellectual powers largely expanded by increase of 

 years. His results were communicated to the Eoyal Society of London, 

 in a paper " On the molecular mobility of gases," t and it is impossible 

 to read this and his original paper " On the law of the diffusion of 

 gases " together, without being struck by the great advance in philo- 

 sophic grasp and breadth of view which had become developed in the long 

 interval between the publication of the two memoirs. These later ex- 

 periments on gas-diffusion were made principally with septa of com- 

 pressed graphite ; and it will be well to preface their consideration by 

 Mr. Graham's own introductory remarks. He observes : 



*The motions of effusiou iiutler pressure, and of spontaneous diffusion, would appear 

 to be alike traceable to the elasticity of the gas itself, exerted under the conditions to 

 ■which it is exposed at the time. 



t Philosophical Transactions, li:'l)3, p. 365. 



