DIFFUSION OF GASES. 301 



now be presented in a general manner ; and some introductory 

 reference must be here made to the well-known physical proper- 

 ties of gases. 1 



803. Diffusion of gases. When two or more gases are brought 

 into contact, spontaneous intermixture takes place. This pro- 

 cess of diffusion, as it is called, goes on even when the gases 

 are very different in specific gravity, and when they are kept 

 externally at perfect rest. Thus if a jar of carbonic acid be 

 placed in connection with a jar of oxygen, the two gases, after 

 a while, will become uniformly commingled. 



Similar commingling of gases also takes place through per- 

 meable substances, such as thin plates of unglazed porcelain, 

 graphite, films of membrane, etc. 



804. Different gases diffuse through a given membrane in 

 different times. The rates of diffusion of different gases at the 

 same temperature and barometric pressure have been shown by 

 Graham to differ nearty in the inverse ratio of the square roots 

 of their densities, thus : 



1 Graham, who made a careful study of the laws which govern gaseous dif- 

 fusion, has given the following clear account of the physical hypothesis, which 

 is now generally received : " A gas is represented as consisting of solid and 

 perfectly elastic spherical particles which move in all directions, and are ani- 

 mated with different degrees of velocity in different gases. Confined in a 

 vessel, the moving particles are constantly impinging against its sides and oc- 

 casionally against each other, and this contact takes place without any loss of 

 motion owing to the perfect elasticity of the particles. If the containing 

 vessel be porous, then gas is projected through the open channels, by the 

 motion described, and escapes. Simultaneously the external air is carried 

 inwards in the same manner and takes the place of the gas which leaves the 

 vessel. To this molecular movement is due the elastic force, with the power 

 to resist compression, possessed by gases. The molecular movement is acceler- 

 ated by heat and retarded by cold, the tension of the gas being increased in the 

 first instance and diminished in the second. Even when the same gas is 

 present both without and within the vessel, or is in contact with both sides 

 of our porous plate, the movement is sustained without abatement molecules 

 continuing to enter and leave the vessel in equal number, although nothing 

 of the kind is indicated by change of volume or otherwise. If the gases in 

 communication be different, but possess sensibly the same specific gravity 

 and molecular velocity as nitrogen and carbonic oxide do, an interchange of 



