1 900]. on the Dynamical Theory of Heat and Light. 397 



" energy of the last-meutioned kiud, and its amount (according to law) 

 "should not be inferior. 



" We are here brought face to face with a fundamental difficulty, 

 " relating not to the theory of gases merely, but rather to general 

 " dynamics. In most questions of dynamics, a condition whose violation 

 "involves a large amount of potential eneigy may be treated as a 

 "constraint. It is on this principle that solids are regarded as rigid, 

 " strings as inextensible, and so on. And it is upon the recognition 

 " of such constraints that Lagrange's method is founded. But the law 

 " of equal partition disregards potential energy. However great may 

 " be the energy required to alter the distance of the two atoms in a 

 "diatomic molecule, practical rigidity is never secured, and the kinetic 

 " energy of the relative motion in the line of junction is the same as if 

 "the tie were of the feeblest. The two atoms, however related, remain 

 " two atoms, and the degrees of freedom remain six in number. 



" What would appear to be wanted is some escape from the 

 " destructive simplicity of the general conclusion." 



The simplest way of arriving at this desired result is to deny the 

 conclusion ; and so, in the beginning of the twentieth century, to lose 

 sight of a cloud which has obscured the brilliance of the molecular 

 theory of heat and light during the last quarter of the nineteenth 

 century. 



[E.] 



