Mr. J. Gill on the Dynamical Theory of Heat. 3 



was taken up by the distinguished engineer M. Hirn of Colmar, 

 who, like M. Seguin at an earlier period, instituted a series of 

 experiments on the actual working of large steam-engines, to as- 

 certain, if possible, the disappearance of heat equivalent to the 

 work done as assumed by the dynamical theory ; but he could 

 not obtain satisfactory proof of the direct disappearance of quan- 

 titative heat from the working steam as an equivalent of the work 

 done. These negative results led to a controversy, in which the 

 correctness of the dynamical theory was ably sustained by Pro- 

 fessor Clausius ; and at his suggestion M. Hirn made a fresh 

 series of experiments on the large scale as before, from which he 

 still could detect no disappearance of the heat supposed to be 

 changed into the work done. His apparent facts were again 

 ably combated by the supporters of the dynamical theory ; and 

 finally, in 1862, M. Hirn published a work in which he admits 

 that his former views were erroneous, and shows himself a con- 

 vert to the dynamical theory, giving a fresh set of experiments 

 on large engines, in which he shows that heat does disappear 

 from steam in the act of performing mechanical work. He argues 

 that it must be so, as work or heat would be produced from no- 

 thing unless we allow that an equivalent of heat has been actually 

 transformed into the work done, or reproduced in the heat deve- 

 loped by friction — and consequently that, in all cases where work 

 has been performed by a hot elastic fluid, the fluid must contain 

 less heat after the work is done than it contained before, the heat 

 which has disappeared being the equivalent of the work done, 

 and in fact transformed into this work. Moreover, when a gas 

 is compressed it becomes hotter ; when it expands against a mo- 

 derated resistance its temperature falls. In a simply dynamical 

 point of view, this case is like the winding up and unwinding of 

 a spring : force from some exterior source is transferred to the 

 compressed fluid, which in expanding back against a moderated 

 resistance to its initial tension, gives out the same amount of 

 force. But how explain the heating and cooling of the gas ? 

 Formerly the explanation was easy, on the supposition that the 

 specific heat of gases varied with the density, the capacity for 

 heat increasing as the density decreased. Regnault's experi- 

 ments showed that the capacity of gases for heat is constant, or 

 nearly so ; consequently the facts of the heating and cooling of 

 a gas by compression and expansion are inexplicable, and really 

 without a cause, unless we allow a direct relation between the 

 work expended or produced and the heating and cooling of the 

 gas. 



It has been supposed that M. Hirn's more recent experiments, 

 described in his Exposition Analytique et Experiment ale, prove an 

 exact proportionality between the work performed by the engine 



B2 



