116 Memoir of Thomas Graham. 



position to tlie force of gravity, but had made this quality of gases 

 the subject of experimental investigation. He infeiTed, 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 evidence, and he failed to 

 perceive the simple law which underlies this whole class of phe- 

 nomena. 



Graham, " on entering on this inquiry, found that gases dif- 

 fuse into the atmosphere with different degrees of ease and 

 rapidity." This was first observed by allowing each gas to dif- 

 fuse from a bottle into the air through a narrow tube in opposi- 

 tion to the solicitation of gravity. Afterwards an observation 

 ol Doeberemer on the escape of hydrogen gas by a fissure or 

 crack in a glass receiver caused him to vary the conditions of 

 f;^.^P?"°^*^^^S' f^^ 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, does not interfere with the molecular motion. 

 Much later, (xraham found in prepared graphite a material far 

 better adapted to this purpose than the plaster, and he used sep- 

 ta of this mineral to confirm his early results, in answer to cer- 

 tain lU-considered criticisms in Bunsen's work on Gasometrj. 

 Ihese septa he was in the habit of calling his "atomic filters.' 

 iiy means of the diffusion tube Graham was able to measure 

 accurately the relative times of diffusion of different gases, and 

 ue found that equal volumes of any two gases interpenetrate each 

 other m times which are inversely proportional to tlie square roots of 

 (h^r respective densities, and this fundamental law was the great- 

 est discovery of our late foreign associate. It is now univer- 

 sally recognised as one of the few great cardinal principles 

 which form the basis of physical science. 

 '- ? shown, on the princij 



should rush into a vacuum with velocities corresponding to the 

 numbers which have been found to express their diifusion 

 . ™ J ' .^^ n' ? ^ ^^^^^^ ^^ experiments on what he calls the 

 /wusion of gases, Graham confinned by trial this deduction 

 ot theory. Iq these experiments a measured volume of the gas 

 was allowed to find its way into the vacuous jar through a mi- 

 nute aperture in a thin metallic plate, and he carefully distin- 

 guished between this class of phenomena and the flowing of 

 gases tlirough capillary tubes into a vacuum, in which case, 



