1 1 4 TRANSPIRA TION AND PART I. 



by 4 the square root of 16. This law has been verified 

 by experiment, and is quite analogous to that which 

 regulates molecular diffusion, but the phenomena are 

 essentially different. It is the gas en masse which par- 

 takes of the movements of effusion, whilst only the 

 molecules or atoms of a gas are affected by the move- 

 ments of diffusion. For that reason the swiftness of 

 the effusion of a gas is many thousand times greater 

 than that of diffusion. The swiftness of the efflux of 

 atmospheric air is as rapid as the velocity of sound. 



The rate of the flow of different gases under constant 

 pressure through capillary tubes into a vacuum, con- 

 stitutes the capillary transpiration of gases. These 

 rates bear a constant proportion to one another, but 

 they are singularly unlike the rates of effusion. They 

 are independent of the material of the tube; they 

 are not governed by specific gravity ; and ' they appear 

 to be in constant relation with no other known property 

 of the same gases ; and they form a class of phenomena 

 remarkably isolated from all else at present known of 



The pores of graphite are so fine that it is incapable 

 either of effusion or transpiration, but it is readily 

 penetrated by means of the molecular or diffusive 

 movements of gases, as appears on comparing the time 

 requisite for the passage of equal volumes of different 

 gases under constant pressure into a vacuum. For 

 oxygen, hydrogen and carbonic acid gas, the times are 

 nearly as the square roots of their densities. 



The atrnolysis or partial separation of mixed gases 

 and vapours of unequal diffusibility, can be effected by 

 allowing the mixture to penetrate through a graphite 

 plate into a vacuum. The amount of separation is in 

 proportion to the pressure, and attains its maximum 

 when the gases pass into a perfect vacuum. One of the 

 results of atmolysis was the concentration of oxygen in 



