120 



HEAT. 



FIG. 73. Joule's Expansion of 

 Air Apparatus. 



to the work done on the gas already in it as successive portions enter 

 and compress it. But since, if the heat developed in one were trans- 

 ferred to the other the original temperature would be regained, the 



experiment shows that mere expan- 

 sion of a gas when no external work 

 is done does not produce a change of 

 temperature. Thus Mayer's mode of 

 calculation was justified. 



Joule's Researches. Already Joule 

 had begun his series of researches and 

 sought to determine the mechanical 

 equivalent by compressing or rare- 

 fying air and equating the heat 

 developed or lost to the work done 

 on or by the air. Here again the 

 calculation depends on the absence 

 of cohesion of the air, and Joule de- 

 vised an experiment apparently inde- 

 pendently, but on the same lines as 

 Gay-Lussac's. Two copper vessels R 

 and E (Fig. 73) were connected by a 



pipe provided with a stop-cock. R was filled with air at about 22 

 atmospheres pressure, E was exhausted, and the two were placed in a 

 vessel containing water. The stop-cock was then opened to allow the 

 air to expand and fill both vessels. On stirring the water and taking its 

 temperature, no appreciable alteration was found. 



When R and E were arranged as in Fig. 74 in separate calorimeters 

 a notable cooling was observed in 

 the vessel surrounding R and a 

 very nearly equal heating in that 

 surrounding E. This last form of 

 experiment was almost identical 

 with Gay-Lussac's.* The method 

 is not capable of great exactness 

 owing to the large heat capacity 

 of the vessels and calorimeter com 

 pared with that of the contained 

 air. But by a method devised later 

 (chap, xviii.) Joule and Thomson 

 showed that there are slight absorp- 

 tions or evolutions of heat in a gas 

 on mere change of volume, though 

 the amounts are too small to affect FIG. 74.- 

 the foregoing calculation of the 

 mechanical equivalent. 



Joule's Researches on the Mechanical Equivalent of Heat. 



At the Cork meeting of the British Association in 1843, Joule gave an 



* Gay-Lussac used two globes, each about 12 litres, and each containing a 

 sensitive spirit thermometer. One was exhausted and the other contained air at 

 atmospheric pressure. On opening the connecting-cock one thermometer rose - 58, 

 the other fell 0-61. 



-Modified Form of Joule's 

 Apparatus. 



