40 THEORY OF HEAT. [CHAP. I. 



liquids. In solid masses, neighbouring molecules would yield to 

 their mutual attraction, if its effect were not destroyed by the 

 heat which separates them. 



This elastic force is greater according as the temperature is 

 higher ; which is the reason why bodies dilate or contract when 

 their temperature is raised or lowered. 



54 The equilibrium which exists, in the interior of a solid 

 mass, between the repulsive force of heat and the molecular attrac 

 tion, is stable ; that is to say, it re-establishes itself when disturbed 

 by an accidental cause. If the molecules are arranged at distances 

 proper for equilibrium, and if an external force begins to increase 

 this distance without any change of temperature, the effect of 

 attraction begins by surpassing that of heat, and brings back the 

 molecules to their original position, after a multitude of oscillations 

 which become less and less sensible. 



A similar effect is exerted in the opposite sense when a me 

 chanical cause diminishes the primitive distance of the molecules ; 

 such is the origin of the vibrations of sonorous or flexible bodies, 

 and of all the effects of their elasticity. 



55. In the liquid or gaseous state of matter, the external 

 pressure is additional or supplementary to the molecular attrac 

 tion, and, acting on the surface, does not oppose change of form, 

 but only change of the volume occupied. Analytical investigation 

 will best shew how the repulsive force of heat, opposed to the 

 attraction of the molecules or to the external pressure, assists in 

 the composition of bodies, solid or liquid, formed of one or more 

 elements, and determines the elastic properties of gaseous fluids ; 

 but these researches do not belong to the object before us, and 

 appear in dynamic theories. 



56. It cannot be doubted that the mode of action of heat 

 always consists, like that of light, in the reciprocal communication 

 of rays, and this explanation is at the present time adopted by 

 the majority of physicists ; but it is not necessary to consider the 

 phenomena under this aspect in order to establish the theory of heat. 

 In the course of this work it will be seen how the laws of equili 

 brium and propagation of radiant heat, in solid or liquid masses, 



