1863.] 617 



atmosphere. The times of penetration of equal volumes of gases 



were now 



Times. 



Oxygen 1 .... 



Air 0-9501 0-9507 



Carbonic acid M860 .... M760 



Hydrogen 0'2505 0'2502 



This penetration of the graphite plate by gases appears to be 

 entirely due to their own proper molecular motion, quite unaided by 

 transpiration. It seems to offer the simplest possible exhibition of 

 the molecular or diffusive movement. This pure result is to be 

 ascribed to the wonderfully fine porosity of the graphite. The inter- 

 stitial spaces, or channels, appear to be sufficiently small to extin- 

 guish transpiration, or the passage of masses, entirely. The graphite 

 becomes a molecular sieve, allowing molecules only to pass through. 



With a plate of stucco, the penetration of gases under pressure is 

 very rapid, and the volumes of air and hydrogen passing in equal 

 times are as 1 to 2'891, which is a number for hydrogen intermediate 

 between its transpiration- volume 2 '04 and diffusion-volume 3 '8, 

 showing that the passage through stucco is a mixed result. 



With a plate of biscuitware, 2'2 millimetres in thickness, the 

 volume of hydrogen rose to 3'754 (air= 1), approaching closely to 

 3'8, the molecular ratio. 



The rate of passage of a gas through graphite appeared also to be 

 closely proportional to the pressure. 



Further, hydrogen was found to penetrate through a graphite plate 

 into a vacuum, with sensibly the same absolute velocity as it diffused 

 into air, establishing the important fact that the impelling force is the 

 same in both movements. The molecular mobility may therefore be 

 spoken of as the diffusive movement of gases ; the passage of gas 

 through a porous plate into vacuum, as diffusion in one direction or 

 single diffusion ; and ordinary diffusion, or the passage of two gases 

 in opposite directions, as double, compound, or reciprocal diffusion. 



Atmolysis. A partial separation of mixed gases and vapours of 

 unequal diffusibility can be effected by allowing the mixture to per- 

 meate through a graphite plate into a vacuum, as was to be expected 

 from the preceding views. As this method of analysis has a practical 

 character and admits of wide application, it may be convenient to 



