HISTORY OF DYNAMICAL THEORY OF HEAT. 339 



ililiiliifti 



dry atmospheric air was forced until a tension of about twenty-two 

 atmospheres at the ordinary temperature of the room was attained. 



The other was exhausted by an air-pump. 

 Being then coupled together, they were im- 

 mersed in a tank containing about sixteen 

 and a half pounds of water, which was stirred, 

 and its temperature taken on a very sensible 

 thermometer, indicating approximately thou- 

 sandths of a degree. The stopcocks were 

 next opened and the air allowed to rush from 

 one reservoir to the other until the tensions 

 were more nearly or quite equal in both. Lastly, the water was again 

 stirred and its temperature carefully noted. A correction was ob- 

 tained after each experiment, by noting the increase of temperature 

 caused by an equal amount of stirring, uninfluenced by any possible 

 effects of the expansion. 



Five experiments upon the thermal effect thus attending the ex- 

 pansion of atmospheric air showed a mean increase in the temperature 

 of the water of 0.0074, while the correction to be applied amounted 

 to 0.0068, leaving a difference quite within the limits of observation 

 by this method. Joule, therefore, concluded that "no change of 

 temperature occurs when air is allowed to expand in such a manner as 

 not to develop mechanical power." 



If this result or property of atmospheric air had been known to 

 Mayer, and construed by him to imply the total absence of a trans- 

 formable, internal store of potential energy in gaseous substance, so 

 that the energy embodying the condition variously styled its pure, 

 real, actual, or sensible heat could only be affected by some external 

 agency, mechanical or thermal, and if the effect upon a thermometer, 

 produced by this condition, had been also known to vary directly with 

 the whole quantity of energy comprising it, the method which he in- 

 dicated would have led to an admissible result. 



But, in reality, Gay-Lussac, from his original experiments, had 

 not come to any very definite conclusions on this point. The tem- 

 perature of each receiver had been found by him to change ; but not 

 using an equivalent device to that of the submerging tank of water, 

 he had not been able to determine, on the whole, whether heat had 

 been lost, or gained, in the expansion. When, therefore, Mayer, in 

 1849, defended his claims by a reference to these first experiments on 

 this point, the answer was available to Joule that, prior to his own 

 researches, the all-important principle assumed had not been recog- 

 nized in science ; and that the results obtained by Gay-Lussac tended 

 only to render the question still more doubtful. 



