HOW LATENT HEAT IS TO BE REGARDED. 34 &amp;gt; 



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 ther^ is consequeatly 

 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 Gay-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 calls 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 



