Mr. J. Gill on the Dynamical Theory of Heat. 115 



But as the water of the calorimeter was, as well as the com- 

 pressed air, of the atmospheric temperature at the commence- 

 ment of the experiment, the fall of temperature in the expanding 

 air will naturally cause it to take heat from the surrounding 

 water until an equilibrium of temperature is established. Hence 

 results in the expanded air what is called a disappearance of heat, 

 which is supposed to be converted into the work produced by 

 the expansion — the real state of the case being that the expanded 

 air has produced the work without any loss of heat, but only a 

 fall of temperature ; and therefore it must be conceived that the 

 operation of compression gave it energy which it did not possess 

 before, and that this energy is for the time of a form very differ- 

 ent from heat. In fact the compressed air contained statical 

 energy like the tension of a wound-up spring, and the principle 

 of conservation is satisfied by the recovery of the mechanical 

 work expended to compress the air in preparing the experiment ; 

 so that loss of heat is not required to strike the balance. 



In the other case of Joule's experiment, where the compressed 

 air expanded tumultuously into the annexed vacuous receiver, 

 no exterior work was produced ; but, as in Hirn's experiment of 

 the tumultuous expansion of high-pressure steam, an increase of 

 the total heat of the fluid must result. i\nd as the expansion in 

 the emitting vessel must be attended with a corresponding fall 

 of temperature (though without loss of heat), the extra heat pro- 

 duced by the tumultuous motion of the exhausting air in the 

 other vessel would compensate this fall of temperature by satis- 

 fying the increased capacity of the expanded fluid for heat, and 

 no reduction of temperature would be perceived in the water of 

 the calorimeter. This state of things is explained by saying that, 

 as no work was performed, no heat disappeared; but the real 

 fact is, that the quantity of heat in the apparatus at the end of 

 the experiment exceeds the initial quantity by the additional 

 heat developed by the tumultuous expansion of the air into the 

 comparatively vacuous receiver. And here, again, the result 

 accords with the principle of conservation, as the mechanical 

 work spent on the compression in preparing the experiment is 

 not recovered, but instead of it we have the equivalent heat. 



In considering of this and of analogous experiments, the candid 

 inquirer must perceive that in no case can he detach or isolate 

 his operations from the mass of physical conditions around him, 

 and that consequently results which might otherwise be regarded 

 as absolute are only relative — a consideration of prime import- 

 ance in such investigations ; for where everything is relative, and 

 no absolute initial and final points can be determined in the ever- 

 changing circuit of elemental phenomena, the principle of con- 

 servation may be strictly preserved where we do not directly 



