DIFFUSION OF GASES THROUGH ABSORBING SUBSTANCES. 503 



The phenomena of diffusion studied by Dr v. Wroblewski are quite distinct 

 from any of these. The septum through which the gas is observed to pass is 

 apparently quite free from pores, and is indeed quite impervious to certain 

 gases, while it allows others to pass. 



It was the opinion of Graham that the substance of the septum is capable 

 of entering into a more or less intimate combination with the substance of the 

 gas ; that on the side where the gas has greatest pressure the process of 

 combination is always going on ; that at the other side, where the pressure of 

 the gas is smaller, the substance of the gas is always becoming dissociated from 

 that of the septum ; while in the interior of the septum those parts which 

 are richer in the substance of the gas are communicating it to those which 

 are poorer. 



The rate at which this diffusion takes place depends therefore on the power 

 of the gas to combine with the substance of the septum. Thus if the septum 

 be a film of water or a soap-bubble, those gases will pass through it most 

 rapidly which are most readily absorbed by water, but if the septum be of 

 caoutchouc the order of the gases will be different. The fact discovered by 

 St Claire-Deville and Troost that certain gases can pass through plates of red- 

 hot metals, was explained by Graham in the same manner. 



Franz Exner * has studied the diffusion of gases through soap-bubbles, and 

 finds the rate of diffusion is directly as the absorption-coefficient of the gas, 

 and inversely as the square root of the specific gravity. 



Stefan t in his first paper on the diffusion of gases has shewn that a law 

 of this form is to be expected, but he says that he will not go further into 

 the problem of the motion of gases in absorbing medium, as it ought to form 

 the subject of a separate investigation. 



Dr v. Wroblewski has confined himself to the investigation of the relation 

 between the rate of diffusion and the pressure of the diffusing gas on the two 

 sides of the membrane. The membrane was of caoutchouc, 0'0034 cm. thick. 

 It was almost completely impervious to air. The rate at which carbonic acid 

 diffused through the membrane was proportional to the pressure of that gas, 

 and was independent of the pressure of the air on the other side of the 



* Pogg. Ann., Bd. 155. 



f Ueber das Gkichgeuncht u. d. Diffusion von Gasgemengen. Sitzb. der k. Akad. (Wien). Jan. 5, 



1871. 



