CHAPTEE VIII 

 DIFFUSION OF GASES 



93. Observations 



BY diffusion we understand the slow mixing of two 

 liquids or gases which were previously separated. Such 

 mixing may be effected by processes of different kinds which, 

 though closely connected, are yet so materially different 

 from each other that it is well to give them different names. 

 In this the terminology of Graham is satisfactory. 



When the two substances are separated by a solid wall 

 which contains one or more narrow openings, the mixing 

 is caused by effusion, the laws of which have already been 

 discussed in Chapter III. 37. This mode of mixing must 

 be distinguished from that which takes place through the 

 pores of a natural or artificial membrane, a porous pot or 

 the like, which for liquids is called osmose, and for gases 

 transpiration ; the slowness with which this transpiration is 

 carried on is a consequence of the internal friction of the 

 gas, dealt with in Chapter VII., which is active within the 

 narrow channels of the porous partition. A process which 

 essentially differs from the last is that which occurs when 

 the substance of the partition is capable of absorbing 

 either one or both of the gases, so that it takes the gas 

 in at one side and gives it out at the other; this process 

 can also be explained by the kinetic hypothesis, but as it is 

 conditioned, not only by the state of motion of the gaseous 

 substance, but also by the movements of the molecules of the 

 liquid or solid partition, we cannot treat of it here. What 

 we have here to describe, viz. diffusion in the narrower 

 sense of the word, is the mixing together of two liquids 

 or gases which directly touch each other without being 



