Dim-BIOS OF OAOi THBOUOH ABSORBING SUBSTANCES. 



When the gM in the two vends are different, each gas is discharged 

 according to this lw independently of the other. 



Tbew phenomena, however, can be observed only when the thickness of the 

 pkte and the diameter of the aperture are very small. 



When this u the case, the distance is very small between a point in the 

 limt TOM til where the mixed gas has a certain composition, and a point in the 

 i^yprnl reaael where the mixed gas has a quite different composition, so that 

 the Telocity of diffusion through the hole between these two points is large 

 r..miiared with the velocity of flow of the mixed gas arising from the difference 

 uf tin- total pressures in the two vessels. 



When the hole is of sensible magnitude this distance is larger, because 

 the region of mixed gases extends further from the hole, and the effects of 

 diffusion become completely masked by the effect of the current of the gas 

 in mass, arising from the difference of the total pressures in the two vessels. 

 In this latter case the discharge depends only on the nature of the gas in 

 the vessel of greater pressure, and on the resultant pressures in the two vessels. 

 It consists entirely of the gas of the first vessel, and there is no appreciable 

 ounter current of the gas of the other vessel. 



Hi -nee the experiments on the double current must be made either through 

 H single very small aperture, as in Graham's first experiment with a glass vessel 

 accidentally cracked, or through a great number of apertures, as in Graham's 

 later experiments with porous septa of plaster of Paris or of plumbago. 



With such septa the following phenomena are observed : 



When the gases on the two sides of the septum are different, but have the 

 same pressure, the reduced volumes of the gases diffused in opposite directions 

 through the septum are inversely as the square roots of their specific gravities. 



If one or both of the vessels is of invariable volume, the interchange of 

 gas will cause an inequality of pressure, the pressure becoming greater in the 

 vessel which contains the heavier gas. 



If a vessel contains a mixture of gases, the gas diffused from the vessel through 

 a porous septum will contain a larger proportion of the lighter gas, and the pro- 

 portion of the heavier gas remaining in the vessel will increase during the process. 



The rate of flow of a gas through a long capillary tube depends upon the 

 viscosity OB internal friction of the gas, a property quite independent of its 

 specific gravity. 



