Mr. J. Gill on the Dynamical Theoinf of Heat. 11 



expansion from cnt-off, I think I can now detect sufficient cause 

 why I could not discover the disappearance of heat which might 

 be fairly attributable to a transformation into the work done by 

 the engine. 



On the other hand, if an isolated mass of steam, for example 

 the steam which fills the full-pressure portion of the stroke at 

 the moment of cut-off, be caused to produce work by its own 

 expansion during the remainder of the stroke, it must lose some- 

 thing equivalent to this work — apparently a proportional quan- 

 tity of its own heat ; and as saturated steam cannot part with 

 any of its heat without suffering a condensation of some of its 

 particles, it should follow that the moderated expansion of steam 

 in the act of doing work should be accompanied by the con- 

 densation of steam-particles throughout the mass, representing 

 an aggregate amount of heat equivalent to the work done. Thus 

 the steam working expansively in the cylinder isolated from ex- 

 terior sources of heat or cold, should assume the condition of 

 a mist from the formation of numerous minute water-particles 

 throughout its mass ; and as the heat which had previously 

 maintained these particles in the state of vapour no longer 

 exists in the cylinder, having, in fact, been transferred or trans- 

 formed into the work done by the expansion, the remaining 

 mass of expanded steam must contain so much less heat, and 

 should accordingly show less in the condenser. 



The grand principle of the Conservation of Energy indicated 

 with indisputable certainty that the work done by a steam-engine 

 must cost its full equivalent to the fire ; but in the absence of any 

 direct proof of the disappearance of this equivalent of heat from 

 the steam in its passage through the engine, 1 was fain to suppose 

 that the heat in passing into the boiler might undergo a transfor- 

 mation into some form of repulsion in the vapour-particles cor- 

 responding in intensity to the temperature and pressure of the 

 steam — and that the work done might be attributed to an equi- 

 valent disappearance of this repulsion with a fall of temperature 

 and tension in the steam, but without loss of its quantitative heat, 

 considered simply as molecular motion. It will perhaps be 

 allowed that this hypothesis was not altogether unreasonable so 

 long as there was no satisfactory experimental proof of the direct 

 disappearance of heat from the steam in its passage through the 

 engine ; and even if I had proved this disappearance of heat, 

 I might still have remained in doubt as to the mode of the 

 change of heat into work so long as I was uncertain whether 

 this process might take place simply on mechanical principles 

 applied to molecular action, as shown in Professor Rankine's 

 hypothesis of molecular vortices. This uncertainty has been 

 removed by the complete confirmation of the results of Regnault's 



