350 THE MECHANICAL EQUIVALENT OF HEAT. 



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 nonelastic bodies ; and I have there 

 calculated the mechanical equivalent 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 purpose 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 demon- 

 strated ad oculos ; and I have reason to believe that the effi- 

 ciency of water-wheels and steam-engines might be easily and 

 advantageously measured by means of a similar calorimoto- 

 rial apparatus. It must, however, be left to the future judg- 

 ment of practical men to decide whether, and to what extent, 

 this method deserves to be preferred to Prony's. 



Heat further becomes latent in certain changes of the state 

 of aggregation of bodies. Since it is a settled fact that both 

 solid and liquid bodies oppose a certain resistance to the sep- 

 aration of their parts, and since in general an expenditure of 

 vis viva is required for the overcoming of mechanical resist- 

 ances, we are led to conclude d priori that whenever the cohe- 

 sion of a body is diminished or done away with, force or heat 

 must become latent ; and this, as is well known, perfectly 

 accords with experience. 



Starting from this point of view, the French physicist 

 Person has attempted to detect a direct quantitative relation 

 between the latent heat of metals, on which he has made a 

 great number of observations, and their cohesion ; but at pres- 

 ent determinations of this kind are beset with almost insur- 

 mountable difficulties. 



The heat which becomes latent in the evaporation of water 



