DIFFUSION OF VAPOURS. 77 



they imbibe various liquids. The absorption of ether, of naptha, 

 of oil of turpentine, which soften the substance of the caout- 

 chouc, without dissolving it, may be referred to. It is likewise 

 always those gases which are more easily liquefied by cold or 

 pressure that pass most readily through both caoutchouc and 

 humid membranes. Dr. Mitchell found that the time required 

 for the passage of equal volumes of different gases, through 

 the same membrane, was 



1 minute, with ammonia. 



2 1 minutes, with sulphuretted hydrogen. 



3 cyanogen. 



5 1 carbonic acid. 



6i nitrous oxide. 



27 i 5 j arsenic tted hydrogen. 



28 olefiant gas. 



374 hydrogen. 



113 oxygen. 



160 carbonic oxide, 



and a much greater time with nitrogen. 



Diffusion of Vapours into air, or spontaneous evaporation. 

 Volatile bodies, such as water, rise into air as well as into a 

 vacuum, and obviously according to the law, by which gases 

 diffuse through each other. Thus if a small quantity of the 

 volatile liquid ether be conveyed into two tall jars standing over 

 water, one half filled with air, and the other with hydrogen gas, 

 the air and hydrogen immediately begin to expand, from the 

 ascent of the ether-vapour in to -them, and the two gases in the 

 end have their volume increased exactly in the same propor- 

 tion. But the hydrogen gas undergoes this expansion in half 

 the time that the air requires ; that is to say, ether- vapour fol- 

 lows the usual law of diffusion in penetrating more rapidly 

 through the lighter gas. 



We are indebted to Dr. Dalton for the discovery that the 

 evaporation of water has the same limit in air as in a vacuum. 

 Indeed the quantity of vapour from a volatile body which can 

 rise into a confined space, is exactly the same, whether that 

 space be a vacuum, or already filled with any air or gas, in any 

 state of rarefaction or condensation. The vapour rises and adds 

 its own elastic force, such as it exhibits in a vacuum, to the 

 elastic force of the other gases or vapours already occupying 

 the same space. Hence, it is only necessary to know what 



