162 THE REALITIES OF MODERN SCIENCE 



the idea of average velocity what we really are con- 

 cerned with is the average square of the velocity, since 

 this measures the average kinetic energy. 



Let us now consider the second condition which must 

 be satisfied if the average velocity of the molecules 

 before and after mutual collisions shall remain constant. 

 We recognize two types of motion which the atoms 

 composing a molecule may have. They may move 

 with reference to each other, just as two persons in 

 walking together may vary somewhat their relative 

 positions. They may, however, preserve the same 

 relative distances apart and yet rotate, one around 

 the other or both about some common point, just as 

 two dancers in waltzing. They may, of course, have 

 a motion which is a sort of a combination of these two 

 internal motions. The pressure on the walls of the 

 container is due to the motion of translation which 

 the molecules have and not to the internal motions 

 of their component atoms. As the molecules collide 

 there might occur transformations of energy whereby 

 these internal motions gained at the expense of the 

 motion of the molecule as a whole. We all have seen 

 illustrations of this phenomenon, as, for example, two 

 skaters, colliding at high speed, may be set spinning 

 if they do not meet head on. If the average velocity 

 of the molecules is not to be altered by such collisions 

 then there must, on the average, be some definite 

 partition of the total kinetic energy of the molecules 

 between translation and internal motions. 



As long as there is no alteration in the total energy 

 of the molecules, such as would occur if the walls of 

 the container were at a different temperature, the 



