194 



Sir J. J. Thomson on Conduction 



energy of a molecule at this temperature ; when x is very- 

 small, I = JNM#, when it is very large, I = XM. 



In the case of solids and liquids, though there may not be 

 collisions between the molecules, the rotation of the molecules 

 endows them with a quasi rigidity, making each molecule 

 behave very much as if its axis of rotation were acted on 

 by a restoring couple proportional to tlie angle through 

 which the axis is displaced and proportional also to the 

 kinetic energy possessed by the body in virtue of its rotation : 

 it behaves in fact very much like a spring whose stiffness is 

 proportional to its kinetic energy. The value of I will be a 

 function of the ratio of XM, the deflecting couple acting on 

 the doublet, to the restoring couple brought into piny when 

 the axis is deflected through unit angle ; as this couple is 

 proportional to iv, the average kinetic energy of the molecules, 

 we have 



I=NMF(XM/w). 



Thus we see that for solids and liquids, as well as for gases, 

 I is a function of MX/mj. 



We need not here go into the question whether the form 

 of the function depends on whether the body is in the solid, 

 liquid, or gaseous state. It is sufficient to notice that 

 whatever the state, when x = 0, ~F(x) = 0, and when x = cc , 

 FO)==l. • 



Thus F(x) will be represented by a curve of the type 

 shown in fig. 1. The force X which occurs in this expression 



Fi>. 1. 



for x is not merely the external electric force acting on the 

 system, the polarized doublets will themselves give rise to 

 strong electric forces, and X is the resultant of such forces 

 and the external electric force. We shall take the force due 

 to the polarization of the doublets as proportional to I and 



