PROFESSOR THOMAS GRAHAM's SCIENTIFIC WORK. 203 



cular or diffusive inovemeut of gases. Tbis pure result is to be ascribed 

 to the wouderfully fine (niiuute) porosity of tbe grapbite. Tbe intersti- 

 tial spaces appear to be suflicieutly small to extiuguisli capillary traus- 

 piration entirely. Tbe grapbite plate is a pneumatic sieve wbicli stops 

 all gaseous matter in mass, and permits molecules only to pass." 



By similarly conducted experiments, a determination was also made 

 of tbe difference of rate, if any, at wbicb hydrogen diffuses through a 

 graphite plate into a vacuum and into atmospheric air. Thus, in one 

 minute of time, the following quantities of hydrogen passed through 

 the graphite plate, in the two cases respectivel}'^ : 



1.289 cubic ceutimeters into a vacuum. 

 1.24:5 cubic coutiiuoters into air. 



These numbers indicate a close approach to eciuality in tbe velocities of 

 passage into a vacuum and into a space of other gas — a yet closer equal- 

 ity being probably attainable by a modified form of experimenting. 



The diffusion of hydrogen into air, as in the above-referred-to experi- 

 ment, is of course accompanied by a diff"usion of air into hydrogen, 

 which bad to be allowed for in calculating out the above result. More- 

 over, Mr. Graham made a special repetition of his early experiments ou 

 interdiffusion, operating with dry instead of moist gas, substituting 

 mercury for water in the diffusion-tube, maintaining a constant pressure 

 by Bunsen's mechanism instead of by a pitcher of water, and using a 

 wafer of graphite instead of a plug of stucco as the porous diaphragm. 

 The theoretical exchange of bj drogen for air being 3.8 volumes for 1, 

 and that of hydrogen for oxygen being 4.0 volumes for one, the ex- 

 changing volumes actually found were 3.S7G and 4.124 respectively. 



Eeferring to the approximatively equally rapid passage of h^'drogeu 

 into a vacuous and aerial space, Mr. Graham remarks as follows ou the 

 subject of interdiffusion : 



" In fine, there can be little doubt left on the mind that the permea- 

 tion through tbe grapbite plate into a vacuum, and the diffusion into a 

 gaseous atmosphere, through tbe same plate, are due to the same inher- 

 ent mobility of the gaseous molecule. They are the exhibition of tbis 

 movement in different circumstances. In interdiffusion we have two 

 gases moved simultaneously through the passages in opposite directions, 

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

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

 gle gas moving in one direction only. Tbe latter case may be assimi- 

 lated to the former if tbe vacuum be supposed to represent an infinitely 

 light gas. It will not invoh^e any error, therefore, to speak of both 

 movements as gaseous diffusion — the diffusion of gas into gas (double 

 diffusion) in tbe 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. 



" Tbe diffusive mobility of the gaseous molecule is a property of mat- 

 ter, fundamental in its nature, and the source of many others. The rate 



