Memoir of Thomas Graham. 



and tied, if suspended in an atmosphere of the same material^ 

 becomes in time greatly distended by the insinuation of this 

 gas through its substance. This effect cannot be the result of 

 simple diffusion ; for it is to be remembered that the thinnest 

 film of water, or of any liquid, is absolutely impermeable to a 

 gas as such ; and, moreover, only the carbonic anhydride passes 

 tiirough the film, very httle or none of the air escaping outward 

 The result depends, first, upon the solution of the carbonic an- 

 hydride by the water on one surface of the film ; secondly, on 

 the evaporation into the air, from the other surface, of the ga^ 

 thus absorbed. Similar experiments were made by Drs. Mitch- 

 ell and Faust, and others, in which gases passed through a lihn 

 of india-rubber, entering into a partial combination with the 

 material on one surface, and escaping from it on the other. 



Graham not only considerably extended our knowledge of 

 this class of phenomena, but also gave us a satisfactory explan- 

 ation of the mode in which these remarkable results are pro- 

 duced. He recognized in these cases the action of a feeble 

 chemical force insufficient to produce a definite compound, but 

 still capable of determining a more or less perfect union, as in 

 tne case of simple solution. He also distinguished the influence 

 ot mass m causing the formation or decomposition of such weak 

 chemical compounds. The conditions of the phenomena under 

 consideration are simply these :— 



First. A material for the septum capable of forming a feeble 

 chemical union with the gas to be transferred. 



feewndly. An excess of the gas on one side of the film and 

 a deficiency on the other. 



Thirdly. Such a temperature that the unstable compound 

 may torm at the surface, where the aeriform constituent is pres- 

 ent in large mass, while it decomposes at the opposite surface, 

 where the quantity is less abundant. 



One of the most remarkable results of Graham's study of this 

 peculiar mode of transfer of aeriform matter through the very 

 substance of solid bodies was an ingenious method of separating 

 the oxygen ft-om the atmosphere. The apparatus consisted 

 simply ot a bag of india-rubber, kept distended by an interior 

 framework, while it was exhausted by a Sprengel pump. Under 

 these circumstances the selective affinity of the c.aoutchouc de- 

 tei-mmes such a difference in the rate of transfer of the two 

 constituents of the atmosphere that the amount of oxygen m 

 the transpired air rises to fortv per cent, and by repeating the 

 f ^^IT.w^lv P^^f oxygen may be obtained. It was at ^f 

 hoped that this naethod might find a valuable application in the 

 arts, but m this Graham was disappointed ; for the same result 



