204 PROFESSOR THOMAS GRAHAM's SCIENTIFIC WORK. 



of dififasibility of any gas has beeu said to be regulated by its specific 

 gravity, tlie velocity of diffusion having beeu observed to vary inversely 

 as the square root of the density of the gas. This is true, but not in 

 the sense of the dififusibility being determined or caused by specific grav- 

 ity. The physical basis is the molecular mobility. The degree of mo- 

 tion -which the molecule possesses regulates the volume which the gas 

 assumes, and is obviously one, if not the only, determining cause of the 

 peculiar specific gravity which the gas enjoys. If it were i^ossible to 

 incBease in a permanent manner the molecular motion of gas, its specific 

 gravity would be altered, and it would become a lighter gas. With the 

 density is also associated the equivalent weight of a gaseous element, 

 according to the doctrine of equal combining volumes." 



In addition to the above two sets of experiments, on the diffusion of 

 a single gas into a vacuum and on the diffusion of one gas into another, 

 a third set of experiments was made on the diffusion of one gas away 

 from another ; or on the partial separation of mixed gases by the pro- 

 cess of atmolysis. The experiments on this subject were conducted in 

 several different ways, but the most striking results were obtained with 

 what Mr. Graham named his tube atmolyser. This instrument consists 

 of one or more lengths of ordinary tobacco-pipe, (conveying the current 

 of mixed gas,) surrounded by a glass tube maintained in a more or less 

 vacuous state by exhaustion with an air-pump. The most difi'usible 

 constituent of the mixed gas passing aAvay in largest proportion 

 through the porous material of the tobacco-pipe, the least diffusible con- 

 stituent becomes concentrated in the residue of gas passing along, and 

 finally delivered by the pipe. By this simple contrivance the proportion 

 of oxygen in ordinary air, transmitted by the tobacco-pipe, was increased 

 from below 21 up to 24.5 percent., as a result of the small superior diffu- 

 sive velocity of nitrogen 1.01, over that of oxygen 0.95. 



In ©xi)eriments made with the far more unequall^^ diffusive gases 

 oxygen and liydrogen, mixed in equal volumes, the i)roportion of oxy- 

 gen transmitted by the tobacco-pipe was increased from the original 50 

 per cent, to 90, and even in some cases, to 95 percent. Electrolytic gas, 

 consisting of 33.3 per cent, oxygen and GG.G per cent, hydrogen, was 

 slowly transmitted through a single tobacco-pipe, in some experiments 

 inclosed in a vacuum, in others exposed to the air. In the vacuum ex- 

 periments the transmitted gas was found to consist of 90.7 per cent, 

 oxygen and 9.3 per cent, hydrogen. In the air experiments, the trans- 

 mitted gas was found to consist of 40.1 per cent, oxygen, 5.5 per cent, 

 hydrogen, and 54.1 per cent. air. In both cases it had lost its explosive 

 character, and acquired the property of re-inflaming a glowing splinter. 



This paper of Mr. Graham's "On the molecular mobility of gases" was 

 supplemented by a communication made to the Chemical Society in 18G4, 

 entitled "Speculative ideas respecting the constitution of matter,"* 

 from which the following extracts are taken : 



* Chemical Society Journal, xvii, p 368. 



