118 HEAT. 



the standard body, and noting the strain produced. Were this done 

 without any consideration of work performed in the transfer, we should 

 then have to measure the kinetic-energy-equivalent of the standard 

 strain of the standard body. But this has not been as yet attempted, 

 and we, therefore, content ourselves with considering the kinetic-energy- 

 equivalent of each case separately. 



Electric and Magnetic energies, as we have already remarked, give 

 very little direct evidence of their existence. They are connecting links 

 imagined to come between some recognised form of energy disappearing 

 and others appearing under circumstances such that we cannot imagine 

 a direct transfer. Hence we look for a fixed rate of exchange between 

 the known forms lost on the one hand in electrification or magnetisation, 

 and the known forms gained on the other hand when the electric and 

 magnetic conditions cease. We shall see when we come to consider the 

 energy relations of the electric current that this fixed rate of exchange 

 is shown to exist. We suppose then that it holds also for the trans- 

 formation into, and out of, the intermediate electric and magnetic condi- 

 tions, which, indeed, we measure by the supposition of this fixed rate. 



Since the other energies are ultimately measured on their trans- 

 formation either into kinetic energy or into heat energy, and since we 

 have fair evidence for fixed rates of transformation into these two it 

 remains to examine the rate of exchange between heat and kinetic 

 energy. We shall proceed to give an account of the experimental 

 evidence, all of which goes to show that here also the rate is fixed. And 

 anticipating this result we may sum up the whole discussion in the 

 following 



Statement of the Principle of the Conservation of Energy. 



Energy is recognised in various forms, and when it disappears in one 

 form it appears in others, and in each case according to a fixed rate of 

 exchange. The total quantity of any energy, measured in terms of any 

 one form, is therefore constant whatever forms it may assume. 



The Rate of Exchange between Mechanical or Kinetic Energy 

 and Heat Energy, or the Mechanical Equivalent of Heat. The 



determination of the rate of exchange depends on the measurement of the 

 work done on some system in which that work results only in a develop- 

 ment of heat, and the simultaneous measurement of the heat so developed. 

 The rate was first determined by calculation from the specific heats of 

 air. This method was first set forth clearly by R. Mayer in a paper 

 published in Liebig's Annalen in 1842. We shall therefore give it first, 

 though a more direct method was shortly afterwards carried out by 

 Joule. 



Mayer's Calculation of the Mechanical Equivalent from the Specific 

 Heats of Air at Constant Pressure and Constant Volume. The Specific Heat 

 of air, as of other gases, at constant pressure exceeds that at constant 

 volume, and if we can assume that the excess is due entirely to the 

 work done in pushing out the surrounding air in expanding, that is if 

 we can assume that no energy is absorbed in merely separating the 

 particles of air, that they possess no appreciable cohesion, this external 

 work is the mechanical equivalent of the difference between the two 

 Specific Heats. Making this assumption, let us suppose that the 

 volume of air is 272'5 cc., which, according to Regnault, increases 1 cc. 



