SECT. III.] PRINCIPLE OF COMMUNICATION. 41 



can be rigorously demonstrated, independently of any physical 

 explanation, as the necessary consequences of common observations. 



SECTION III. 

 Principle of the communication of heat 



57. We now proceed to examine what experiments teach us 

 concerning the communication of heat. 



If two equal molecules are formed of the same substance and 

 have the same temperature, each of them receives from the other 

 as much heat as it gives up to it ; their mutual action may then be 

 regarded as null, since the result of this action can bring about no 

 change in the state of the molecules. If, on the contrary, the first 

 is hotter than the second, it sends to it more heat than it receives 

 from it ; the result of the mutual action is the difference of these 

 two quantities of heat. In all cases we make abstraction of 

 the two equal quantities of heat which any two material points 

 reciprocally give up ; we conceive that the point most heated 

 acts only on the other, and that, in virtue of this action, the first 

 loses a certain quantity of heat which is acquired by the second. 

 Thus the action of two molecules, or the quantity of heat which 

 the hottest communicates to the other, is the difference of the two 

 quantities which they give up to each other. 



58. Suppose that we place in air a solid homogeneous body, 

 whose different points have unequal actual temperatures ; each of 

 the molecules of which the body is composed will begin to receive 

 heat from those which are at extremely small distances, or will 

 communicate it to them. This action exerted during the same 

 instant between all points of the mass, will produce an infinitesi 

 mal resultant change in all the temperatures : the solid will ex 

 perience at each instant similar effects, so that the variations of 

 temperature will become more and more sensible. 



Consider only the system of two molecules, m and n, equal and 

 extremely near, and let us ascertain what quantity of heat the 

 first can receive from the second during one instant : we may 

 then apply the same reasoning to all the other points which are 



