Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 517 



view, the phenomena wherein heat becomes 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. Therefore as soon 

 as we know how much of the heat that has become 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. IS T ow Gay-Lussac has proved by experiment 

 that the specific heat of a gas undergoes no sensible alteration 

 in flowing from a containing vessel into a vacuum. Hence it 

 follows that a gaseous body opposes no perceptible resistance to 

 the separation of its particles, 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 becomes latent by the expansion of a gas is there- 

 fore 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 pro- 

 duced 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 men- 

 tioned I have shown that this law of nature is capable of a very 

 much wider application, and that the heat which becomes latent 

 in the expansion of a gas reappears again in every case, if the 

 vis viva thereby produced is employed to generate heat, whether 

 by the compression of air, by friction, or by the impact of non- 

 elastic bodies; and I have there calculated the mechanical equi- 

 valent of heat upon principles of which the accuracy cannot be 

 disputed. I also measured at that time, by way of control, the 

 heat produced in the manufacture of paper in Holland, and 

 compared it with the working force expended, and so found a 

 sufficient degree of concordance between the two quantities. I 

 have recently, moreover, succeeded in constructing, for the pur- 

 pose of the direct determination of the mechanical equivalent of 

 heat, a very simple thermal dynamometer on a small scale, with 

 which the truth of the principle in question can be demonstrated 

 ad oculoSj and I have reason to believe that the efficiency of 



