AND MODERN PHYSICS. 143 



Threo casos arc considered by Mr. Bryan, in which 

 tho law of distribution fails for rigid molecules: tho 

 first is when tho molecules have all, in addition to 

 their velocities of agitation, a common velocity of 

 translation in a fixed direction ; the second Is when tho 

 gas has a motion of uniform rotation about a fixed 

 axis ; while tho third is when each molecule has an 

 axis of symmetry. In this last caso the forces acting 

 during a collision necessarily pass through the axis 

 of symmetry, tho dngular velocity, therefore, of any 

 molecule about this axis remains constant, the 

 number of molecules having a given angular velocity 

 will remain the same throughout the motion, and tho 

 part of the kinetic energy which depends on this 

 component of tho motion will remain fixed, and will 

 not como into consideration when dealing with tho 

 equal partition of tho energy among the various 

 degrees of freedom. 



Such a moleculo has five, and not six, degrees of 

 freedom; threo quantities are needed to determine tho 

 position of its centro of gravity, and two to fix the 

 position of tho axis of symmetry. 



In this caso, then, as Uoltzmann points out, in tho 

 expression for tho ratio of tho specific heats, wo must 

 have n. equal to 5, and henco 



o o 



y i* 1 + * 1 + I c* H 



n 5 



agreeing fairly with tho value found for air and 

 various other permanent gases. 



For cases, then, in which wo consider each atom 

 as a singlo rigid body, tho Boltzinann - Maxwell 



