DIFFUSION OF GASES. 323 



If carbonic acid is left to stand in an open vessel, and 

 after some time a burning splinter be introduced into 

 the vessel, it will continue to burn, thus proving that 

 the gas has escaped from the vessel. The carbonic 

 acid, though heavier, has diffused into the air above it, 

 although the latter is lighter, and the diffusion has pro- 

 ceeded far more rapidly than it proceeds between liquid 

 bodies ; for, half an hour after filling the vessel, all 

 carbonic acid has escaped. 



The diffusion between different gases also takes place 

 nore rapidly than between liquids if they are separated 

 )y a porous partition ; both classes of bodies are, how- 

 iver, alike in this, that the lighter substance passes more 

 apidly through the partition than the heavier. It is 

 isual to denote this kind of action between gases by 

 he term 'diffusion,' while the analogous phenomenon 

 ri the case of liquids is commonly spoken of as ' endos- 

 aose.' 



A partition made of plaster of Paris is very suitable 



Dr experiments on diffusion. A glass funnel, having 



,s wide mouth closed by a plate of plaster of Paris, may 



e filled with carbonic acid, by placing it, stem upwards, 



pon a glass plate, and inserting through the stem a 



arrow tube, which is attached to the india-rubber tube 



:' the apparatus for making the gas ; this tube must be 



ng enough to reach to the widest part of the funnel, 



shown in fig. 207, A. When the funnel may be sup- 



>sed to be full of gas, it is lifted together with the 



i^ass plate, the end dipped into water (fig. 207, B), and 



ie glass plate is removed. Carbonic acid will now 



}ss out through the plaster wall, but the lighter air 



wing inwards with greater velocity, increases the 



Y 2 



