32 THEORY OF HEAT. [CHAP. I. 



38. To this explanation of the chief results of observation, a 

 general remark must be added on equilibrium of temperatures; 

 which consists in this, that different bodies placed in the same re 

 gion, all of whose parts are and remain equally heated, acquire also 

 a common and permanent temperature. 



Suppose that all the parts of a mass M have a common and 

 constant temperature a, which is maintained by any cause what 

 ever: if a smaller body m be placed in perfect contact with the 

 mass M, it will assume the common temperature a. 



In reality this result would not strictly occur except after an 

 infinite time : but the exact meaning of the proposition is that if 

 the body m had the temperature a before being placed in contact, 

 it would keep it without any change. The same would be the case 

 with a multitude of other bodies n, p, q, r each of which was 

 placed separately in perfect contact with the mass M : all would 

 acquire the constant temperature a. Thus a thermometer if suc 

 cessively applied to the different bodies m, n,p, q, r would indicate 

 the same temperature. 



39. The effect in question is independent of contact, and 

 would still occur, if every part of the body m were enclosed in 

 the solid M, as in an enclosure, without touching any of its parts. 

 For example, if the solid were a spherical envelope of a certain 

 thickness, maintained by some external cause at a temperature a, 

 and containing a space entirely deprived of air, and if the body m 

 could be placed in any part whatever of this spherical space, with 

 out touching any point of the internal surface of the enclosure, it 

 would acquire the common temperature a, or rather, it would pre 

 serve it if it had it already. The result would be the same for 

 all the other bodies n, p, q, r, whether they were placed separately 

 or all together in the same enclosure, and whatever also their sub 

 stance and form might be. 



40. Of all modes of presenting to ourselves the action of 

 heat, that which seems simplest and most conformable to observa 

 tion, consists in comparing this action to that of light. Mole 

 cules separated from one another reciprocally communicate, across 

 empty space, their rays of heat, just as shining bodies transmit 

 their light. 



