88 



DIFFUSION 



virtually zero, and that in the bottle equivalent to one atmosphere, the inward 

 diffusion of carbon dioxide gas results in a considerable distension of the 

 balloon. 



In the analysis of diffusion phenomena it is often clarifying to speak of 

 the partial pressure of a gas as its diffusion pressure. Solutes and liquids 



* 



t 



^ 



c=^ 



Fig. 17. Apparatus for demonstrating the development of pressure during diffusion 

 of gases. {A) porous clay cup, (B) glass tube, (C) vessel of colored water. 



may also be considered to possess a diffusion pressure, although the existence 

 of such a physical quantity is less evident in such systems than in gases. Hence 

 the concept that diffusion is the movement of the molecules of a substance 

 from a region of its greater to a region of its lower diffusion pressure is often 

 more satisfactory than the interpretation of diffusion phenomena in terms of 

 concentration differences. 



