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



water-wheels and steam-engines might be easily and advan- 

 tageously measured by means of a similar calorimotorial appa- 

 ratus. It must, however, be left to the future judgment of prac- 

 tical 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 separation 

 of their parts, and since in general an expenditure of vis viva 

 is required for the overcoming of mechanical resistances, we 

 are led to conclude a priori that whenever the cohesion 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 present determina- 

 tions of this kind are beset with almost insurmountable difficul- 

 ties. 



The heat which becomes latent in the evaporation of water has 

 been considered from quite a similar point of view by Holtzmann 

 in his important memoir " On the Heat and Elasticity of Gases 

 and Vapours." Starting from the principle that elevation of tem- 

 perature is equivalent to the raising of a weight, this philosopher 

 has likewise calculated the mechanical equivalent of heat from 

 the quantity of heat which becomes latent by the expansion of a 

 gas ; and he very rightly conceives of the latent heat of steam as 

 made up of two parts, whereof one, the smaller, is expended in 

 overcoming the opposing pressure of the atmosphere, and can 

 hence be easily calculated by means of the mechanical equivalent 

 of heat, while the remaining part, the amount of which can also 

 be calculated, is what Holtzmann calls the heat required to destroy 

 the cohesion of the water. In all steam-engines this latter por- 

 tion is wasted, and Holtzmann calculates from these data the 

 superior efficiency of high-pressure compared with low-pressure 

 engines*. 



If the view here taken of the latent heat of fusion and evapo- 

 ration is correct, heat must also become latent when hard bodies 

 are reduced to powder; and when such substances pass into 

 the liquid condition from a state of fine division, they must 

 absorb a smaller quantity of heat than when they are liquefied 

 without previous comminution. A few experiments that I have 



* The engines which give the greatest useful effect must be those in 

 which the steam receives an addition of heat during its expansion. 



