84 THIRD SERIES ON RAREFACTION. 



mechanical character of the thermal changes caused by 

 compression and rarefaction of air. The results fulfilled 

 his anticipation most emphatically, there being no sensible 

 change in the temperature ol water. 



He then varied the experiment by immersing each of 

 the receivers in a separate vessel, and the connecting piece 

 in a third vessel, when he found that 2*36 units of heat had 

 been abstracted from the receiver in which the air had been 

 expanded, 0*31 from the can in which the connecting piece 

 was immersed, and that 2*38 had been produced in the 

 other receiver, leaving O'Oi units of heat for what were lost 

 in the short lengths of pipe between the cans. 



These two experiments, so simple and yet so crucial,, 

 as to the convertibility of heat into work, were not easily 

 available for determining the mechanical equivalent of 

 heat ; but they afforded such forcible evidence of the purely 

 mechanical character of the elasticity of air, as to start 

 immediately, in the mind of Joule, the now established 

 dynamical theory of gases. 



The third of these series of experiments was again 

 devoted to the evaluation of the equivalent. It is, in fact,, 

 a modification of his second experiment adapted to that 

 evaluation for which it is much better suited than his first 

 series. Here he uses one receiver with a cock leading into 

 a long pipe coiled up and immersed in the same calorimeter 

 as the receiver. The open end of the pipe passes into a 

 pneumatic trough for measuring the volume of air. On the 

 cock being slightly opened the air passes slowly out of the 

 receiver through the pipe and escapes at the temperature 

 of the water doing work only against the pressure of the 

 atmosphere. Thus the work that was done by the escaping 

 air was simply the product of its volume, multiplied by the 



