198 On supposed Objections to the Dynamical Theory of Heat. 



and compresses the air into half its volume, the temperature 

 being kept constant, each particle of the air still strikes the bot- 

 tom of the piston with exactly the same energy as before ; but 

 the number of particles now striking the piston is doubled. 

 Hence the elasticity of the air is also doubled. When the abso- 

 lute temperature is doubled, for example, the elasticity is like- 

 wise doubled, because in this case each particle gives two strokes 

 for one that it gave before. 



Let us now refer to his next difficulty. Suppose a cylinder 

 containing air under a pressure of two atmospheres is allowed to 

 double its volume by rushing into another cylinder which had 

 previously been made a vacuum. The particles of the air rush- 

 ing into the vacuum with great velocity must strike against the 

 sides of the cylinder^and thus produce heat. But it is found, 

 from the experiments of Dr Joule and others, that the air after ex- 

 pansion is at the same temperature as before it. This fact, he 

 says, can only be explained upon the supposition that the specific 

 heat of the air increases as it expands. But such a conclusion 

 by no means follows. What is it that drives the particles of the 

 air out of cylinder A into cylinder B ? The elasticity of the air 

 of course. Or, in other terms, the vis viva of the moving par- 

 ticles of the air as they enter the cylinder B, is derived from that 

 form of molecular motion called heat possessed by the particles 

 in the cylinder A. The particles in the cylinder A must then 

 lose heat in consequence. The vis viva of the moving particles 

 in the cylinder B becomes converted into the molecular motion 

 of heat when the particles strike against the sides of the cylinder, 

 and consequently the temperature must rise. But the rise of 

 temperature in cylinder B is exactly counterbalanced by the cor- 

 responding fall of temperature in cylinder A. Hence the tem- 

 perature of the air must remain unchanged. If the specific heat 

 of the air increased with the diminution of density, as Mr. Gill 

 supposes, then the temperature of the air after expansion ought 

 to be less than before it, a conclusion contradicted by experiment. 

 I am, Sir, 



Your most obedient Servant, 



Glasgow, February 8, 1864. James Croll. 



[Out of deference to a practical experimenter, we gave Mr. Gill 

 an opportunity of stating his objections to the Dynamical Theory of 

 Heat. But we do not think that it would be desirable to prolong 

 this discussion Eds. "J 



