16 The Uev. J. M. Heath on the Production of Heat 



heating it is y?;' = 26275, giving an increase of 51 cubic-foot 

 pounds. 



In warming the gas 1° we expended an energy of 183*6 

 cubic-foot pounds ; and if no change had been made in the 

 volume, the energy of the warmer air under the same volume 

 Avould have been 183"6 greater than that of the cooler. But 

 by the reduction of the volume by '062 there has been a loss 

 of -062 X 2130-8, or 132*1 cubic-foot pounds, reducing the 

 final gain from 183*6 to 51, which was the actual gain as 

 found above. 



No heat has ever been generated in a gas spontaneously and 

 without communication with external bodies, except under the 

 condition of the disturbance of the equilibrium of the forces, 

 as explained above. As long, therefore, as the forces remain 

 in equilibrium (as they do, for example, during the uniform 

 rise or fall of the barometer), any amount of condensation or 

 expansion may take place without affecting the temperature. 

 This is contrary to universal belief. But it is true for all that. 

 It is not from thermocli/nainists that we should have expected to 

 hear that all heat is generated by forces ivhich are in equilibriuyn 

 with each other. This assertion, now, as it is believed, made 

 for the first time, of the existence of what may be called 

 thermostatical condensation and expansion, may be put beyond 

 all doubt by reference to the two equations we established in 

 the beginning of this article, as founded on experience and 

 governing all the cases where heat is really produced dynami- 

 cally. Those equations were 



vdp = '001kdt, 

 pdv = -006Mt, 

 and a third, which may be derived from these two, 

 5vdp = Ij^dv. 



From these it appears that no heat is spontaneously generated 

 in the process of equilibrating a dynamical disturbance — first, 

 in the case where dp = 0, secondly where dv = 0, and thirdly 

 where vdp=pdv, or, indeed, where vdp equals any thing else 

 than 1*4 pdv. That is, no heat is gained or lost by the con- 

 densation or expansion of a gas under constant pressure , nor 

 by any possible variation of the pressure tvhile the volume is 

 constant, nor by any variation of volume and pressure together 

 during which the pressure varies inversely as the volume. 



Lastly, it appears from these principles that a gas may be 

 heated during expansion, or cooled daring condensation. The 

 equation 



rdp = -007 kid, 



