232 PROCEEDINGS OP THE AMERICAN ACADEMY 



does not interfere with the molecular motion. Much later, Graham 

 found in prepared graphite a material far better adapted to this pur- 

 pose than the plaster, and he used septa of this mineral to confirm his 

 early results, in answer to certain ill-considered criticisms in Bunsen's 

 work on Gasometry. These septa he was in the habit of calling I113 

 " atomic filters." By means of the diffusion tube Graham was able to 

 measure accurately the relative times of diffusion of different gases, and 

 he found that equal volumes of any two gases interpenetrate each other 

 in times which are inversely proportional to the square roots of their re- 

 spective densities, and this fundamental law was the greatest discovery 

 of our late Foreign Associate. It is now universally recognized as 

 one of the few great cardinal principles which form the basis of Physi- 

 cal Science. 



It can be shown, on the principles of pneumatics, that gases should 

 rush into a vacuum with velocities corresponding to the numbers which 

 have been found to express their diffusion times ; and, in a series of ex- 

 periments on what he calls the " Effusion" of gases, Graham confirmed 

 by trial this deduction of theory. In these experiments a meas- 

 ured volume of the gas was allowed to find its way into the vacuous jar 

 through a minute aperture in a thin metallic plate, and he carefully 

 distinguished between this class of phenomena and the flowing of gases 

 through capillary tubes into a vacuum, in which case, however short 

 the tube, the effects of friction materially modify the result. This last 

 class of phenomena Graham likewise investigated, and designated by 

 the term " Transpiration." 



While, however, it thus appears that the results of Graham's inves- 

 tigation were in strict accordance with Dalton's theory, it must also be 

 - evident that Graliam was the first to observe the exact numerical re- 

 lation which obtains in this class of phenomena, and that all-impor- 

 tant circumstance entitles him to be regarded as the discoverer of the 

 law of Diffusion. The law, however, as first enunciated, was purely 

 empirical, and Graham himself says that something more must be as- 

 sumed than that gases are vacua to each other, in order to explain all 

 the phenomena observed ; and according to his original view this rep- 

 resentation of the process was only a convenient mode of expressing 

 the final result. Such has proved to be the case. 



Like other great men, Graham built better than he knew. In the 

 progress of Physical Science during the last twenty-five years, two 

 principles have become more and more conspicuous, until at last they 



