Intelligence and Miscellaneous Articles. 445 



A THEORETIC AND EXPERIMENTAL DEMONSTRATION OF THE DE- 

 FINITION, " THE TEMPERATURE OF A BODY IS REPRESENTED 

 BY THE LENGTH OF THE THERMAL OSCILLATION OF ITS MO- 

 LECULES." BY R. PICTET. 



If we admit that heat is only the manifestation pure and simple 

 of the molecular forces with which the constituent particles of 

 bodies are endowed, we must necessarily admit also that the me- 

 chanical work taken up by the thermal motion must displace the 

 particles from their position of equilibrium and make them move in 

 trajectories of an elliptic form, the amplitude of which will be pro- 

 portional to the work consumed. 



At absolute zero there is no oscillation, the cohesion is maximum ; 

 at a certain temperature, fixed for each body, the oscillation will 

 be maximum, and the body, being disaggregated, will dissolve ; the 

 molecules will be sufficiently apart to be out of the conditions of 

 stable equilibrium. 



In this hypothesis absolute contact of material particles is ren- 

 dered impossible by the action of the aether ; for we admit that the 

 attraction of matter for the aether does not follow the same law as 

 the attraction of matter for matter : for short distances the attrac- 

 tion of matter for the aether prevails over the attraction of matter 

 for itself. 



Under these conditions the repulsive forces are useless ; there 

 would exist in nature only attractive forces. 



On these bases let us consider the action produced by external 

 work supplied to a body supposed at absolute zero. Each molecule 

 will begin to vibrate and oscillate from an extreme exterior position 

 to another position, an interior limit. The evident result of this 

 molecular motion will be to increase the volume of the body in 

 proportion to the mean length of the oscillations of its elementary 

 particles. 



The coefficient of dilatation will therefore be in accordance either 

 with the number of molecules contained in the body, or with the 

 volume in which those molecules are contained, or, lastly, with the 

 physical forces involved in thermal motion. 



Now the two following postulates can be admitted: — 

 The laws of the attraction of matter for matter are absolutely general 

 and universal. 



The phenomena of the disaggregation of bodies are subject to those 

 laws. 



These being admitted, let N be the number of molecules con- 

 tained in unit length of a solid body, V and I the lengths of oscilla- 

 tion corresponding to the temperatures t' and t ; let a be the coeffi- 

 cient of dilatation of the solid body. We shall evidently have the 

 following equality : — 



adt. 



J>-f 



Now N is defined by the density and atomic weight of the solid 

 body. 



Phil. Mag. S. 5. Vol. 7. No. 45. June 1879. 2 L 



