76 VAPORIZATION. 



into a diffusion tube with velocities denoted by the following 

 numbers, 1277 ? 623, 302, according as the diffusion tube is 

 filled with hydrogen, with carbonic acid, or with chlorine gas. 

 Now, if the air were rushing into a vacuum in all these cases, 

 why should it not always enter it with the same velocity ? 

 Something more, therefore, must be assumed than that gases 

 are vacua to each other, in order to explain the whole phe- 

 nomena observed in diffusion. 



Passage of gases through membranes. In connexion with 

 diffusion, the passage of gases through humid membranes 

 may be noticed. If a bladder, half filled with air, with its mouth 

 tied, be passed up into a large jar filled with carbonic acid 

 gas, standing over water, the bladder, in the course of twenty- 

 four hours becomes greatly distended, by the insinuation of the 

 carbonic acid through its substance, and may even burst, while 

 a very little air escapes outwards from the bladder. But this is 

 not simple diffusion. The result depends upon two circum- 

 stances ; first, upon carbonic acid being a gas easily liquefied 

 by the water in the substance of the membrane, the carbonic 

 acid penetrates the membrane as a liquid ; secondly, this liquid 

 is in the highest degree volatile, and, therefore, evaporates very 

 rapidly from the inner surface of the bladder into the air con- 

 fined in it. The air in the bladder comes to be expanded in 

 the same manner as if ether or any other volatile fluid was ad- 

 mitted into it. The phenomenon was observed by Dalton in 

 its simplest form. Into a very narrow jar, half filled with 

 carbonic acid gas over water, he admitted a little air. The 

 air and gas were accidentally separated by a water bubble, 

 and thus prevented from intermixing. But the carbonic gas 

 immediately began to be liquefied by the film of water, and 

 passing through it, evaporated into the air below. The air 

 was in this way gradually expanded, and the water bubble as- 

 cended in the tube. Here the particular phenomenon in ques- 

 tion was observed to take place, but without the intervention of 

 membrane. It is to be remembered that the thinnest film of 

 water or any liquid is absolutely impermeable to a gas as such. 



In the experiments of Drs. Mitchell and Faust and others, in 

 which gases passed through a sheet of caoutchouc, it is to be 

 supposed that the gases were always liquefied in that substance, 

 and penetrated through it in a fluid form. Indeed few bodies 

 are more remarkable than caoutchouc for the avidity with which 



