CHAPTER II 

 DIFFUSION AND DIFFUSION TENSION 



I. GASES 



a) Simple gases. — As has been indicated already, it is a 

 fundamental property of all gases that they tend to fill com- 

 pletely any vessel in which they may be inclosed. Thus, if 

 a cubic centimeter of oxygen is measured out at ordinary 

 temperature and at atmospheric pressure, and is then passed 

 into a sealed vacuum chamber, it will completely fill the 

 chamber, no matter how large the latter may be. This 

 process of expansion is called difEusion. Of course, in dif- 

 fusing, the particles of which the gas is composed become 

 distributed throughout a greater space, and hence the gas 

 becomes less dense. This property is often stated as follows: 

 "The particles of gases tend to separate indefinitely." 



Because of this tendency to expand, an outward pressure, 

 called gas pressure, is exerted by a gas upon the walls of 

 any chamber in which it may be confined. Gas pressure is 

 supposed to be caused by the continuous bombardment of 

 the walls of the inclosing vessel by the vibrating gas par- 

 ticles. If a gas be inclosed in a chamber with elastic walls, 

 the size of the chamber will depend upon the number of 

 particles of gas present (t. e., its concentration) and upon 

 the kinetic energy of the particles themselves (i. e., its tem- 

 perature). Thus, for any temperature and amount of gas, 

 the distension of such a chamber will cease when the inward 

 pressure, due to the resilience of the walls and to the pres- 

 sure of the surrounding atmosphere (unless the chamber be 

 in a vacuum), becomes equal to the outward pressure, due 

 to the gas. 



