88 Mr. J. Gill on the Dynamical Theory of Heat, 



dered equivalent to the work expended, as by allowing the air 

 to expand back under moderated resistance, restoring to it the 

 heat it had lost, the work done in the compression would be 

 recovered. Now suppose a communication to be made between 

 the two cylinders — the air will rush into the vacuum, and as the 

 contents of the two vessels now filled by the expanded air are 

 equal to the volume which the air originally occupied, it is evi- 

 dent that the pressure will also be equal to what it was before 

 the compression, namely atmospheric pressure. It is clear that 

 no power can be obtained from the expanded air, as it is in equi- 

 librium with the surrounding atmosphere ; but the energy which 

 might have been availed of by allowing the atmosphere to fall 

 into the vacuum, and thus produce work, has disappeared. It 

 is impossible to conceive of the annihilation of energy, and we 

 must therefore suppose that it has taken some other shape in 

 the confined air. The dynamical theory does not allow the ex- 

 istence of energy in elastic fluids under any other form than that 

 of molecular motion or heat, consequently there should be more 

 heat in the expanded air ; but as the experiments of Gay-Lussac 

 and Joule have proved that in the expansion of air into a vacuum 

 the temperature of the mass remains constant, we are forced to 

 admit that the specific heat of the expanded air is greater than 

 that of the air in a compressed state : in other words, the sus- 

 ceptibility of air to the influence of heat as affecting the ther- 

 mometer increases with increase of density. 



According to the dynamical theory, a common non-condensing 

 steam-engine, working without expansion, utilizes only about -^ 

 of the theoretical power of the steam. By the addition of a con- 

 denser this effect is doubled, or the engine gives about ~ of 

 useful effect. Such were the deductions published by Regnault, 

 conveying to abstract theorists the idea that the steam-engine is 

 one of the most imperfect of prime movers. Some of the results 

 obtained by M. Hirn from the practical working of large steam- 

 engines were as high as ±, none lower than y 1 ^ ; and ^ may be 

 taken as a possible degree of practical efficiency — better certainly 

 than Regnault's deductions, but still showing that the steam- 

 engine under the most favourable circumstances is so far from 

 perfection as to throw away J of the theoretical power of the 

 steam. However, the marked discrepancy between the theore- 

 tical deductions and the practical results showed that something 

 was wrong in the theoretical calculation ; and the excess of work 

 obtained in Hirn's results has been accounted for by supposing 

 that some of the steam condenses in the act of expansion in the 

 working of the engine, and that this condensation represents the 

 heat converted into work beyond the theoretical calculation which 

 supposed all the steam to pass into the condenser in the vaporous 



