14 GENERAL PROPERTIES OF LIVING TISSUES 



are assumed to be perfectly elastic, so that the 

 motion of a molecule is not lost when it collides 

 with another molecule or strikes the wall; the 

 direction of the flight is changed but the swift- 

 ness is unimpaired. The speed of the molecules 

 of a gas is inversely proportional to the square 

 root of its density. At C. the molecule of 

 oxygen moves at the rate of almost eighteen 

 miles per minute. 



Diffusion of gases. The molecules of two 

 gases moving in the same space will intermix 

 or " diffuse," but the rate will be surprisingly 

 slow, for at ordinary pressures the molecules of 

 gases are so near together that they cannot move 

 far without colliding and rebounding. Their prog- 

 ress in any one direction is thus greatly hindered. 



Every particle of matter attracts every other 

 particle. Van der Waal assumes that with gases 

 this attraction is proportional to the square of 

 the density of the gas. Where the density is 

 slight, that is, where there are few molecules in 

 proportion to the space in which they move, this 

 attraction need not be taken into account, but 

 where the confining space is small, thus crowd- 

 ing the molecules together, this attraction be- 

 comes important. It is still more important in 

 the case of liquids, for in liquids the molecules 

 are much nearer than in a gas. 



