HOW LATENT HEAT IS TO BE REGARDED. 349 



how, according to my view, the phenomena wherein heat be- 

 comes latent are to be regarded. 



If heat is communicated to a gas retained under constant 

 pressure, the free heat of the gas is increased, and at the 

 same time a calculable quantity of heat becomes latent; the 

 gas is thereby caused to expand, and there is consequently 

 produced an amount of vis viva proportional to the pressure 

 and to the space through which expansion takes place. There- 

 fore as soon as we know how much of the heat that has be- 

 come latent is to be attributed to the expansion of the gas, we 

 know also the amount of the remainder of the latent heat 

 corresponding to the vis viva produced. Now Glay-Lussac 

 has proved by experiment that the specific heat of a gas un- 

 dergoes no sensible alteration in flowing from a containing ves- 

 sel into a vacuum. Hence it follows that a gaseous body op- 

 poses no perceptible resistance to the separation of its parti- 

 cles, and that the rarefaction of a gas does not of itself (that 

 is, when it occurs without any evolution of force) cause any 

 heat to become latent. The total quantity of heat which be- 

 comes latent by the expansion of a gas is therefore to be taken 

 as the equivalent of the vis viva produced. 



It results from the principle of the indestructibility of 

 heat a principle which no one jcalls in question that the 

 quantity of heat which has thus become latent must again 

 become free when heat is in any way produced at the expense 

 of the acquired vis viva of motion. Motion is latent heat, 

 and heat is latent motion. 



The celebrated law of Dulong, that the amount of heat 

 produced by the compression of a gas is dependent on the 

 amount of force expended, and not upon the chemical nature, 

 tension, or temperature of the gas, is a special application of 

 the above general principle. But in the communication so 

 often mentioned I have shown that this law of nature is capa- 

 ble of a very much wider application, and that the heat which 

 becomes latent in the expansion of a gas reappears again in 



