Dr. J. R. Mayer on the Mechanical Equivalent of Heat* 515 



one another by saying (Phil. Mag. S. 4. vol. xxiv. p. 372) that 

 they are " different forms under which one and the same object 

 makes its appearance." At the same time I have expressly 

 guarded myself from making the certainly plausible, but un- 

 proved and, as it seems to me, hazardous deduction that thermal 

 phenomena are to be regarded as merely phenomena of motion. 

 The following is what I said upon this point {he. cit. p. 376): — 



"But just as little as the connexion between falling- force and 

 motion authorizes the conclusion that the essence of falling-force 

 is motion, can such a conclusion be adopted in the case of heat. 

 We are, on the contrary, rather inclined to infer that before it 

 can become heat, motion — whether simple, or vibratory as in 

 the case of light and radiant heat, &c. — must cease to exist as 

 motion." 



The relation which, as we have seen, subsists between heat and 

 motion has regard to quantity, not to quality ; for (to borrow the 

 words of Euclid) things which are equal to one another are not 

 therefore similar. Let us beware of leaving the solid ground of 

 the objective, if we would not entangle ourselves in difficulties of 

 our own making. 



In the mean time it at least results from the foregoing consi- 

 derations that the phenomena of heat, electricity, and magnetism 

 do not owe their existence to any peculiar fluids ; and the im- 

 materiality of heat, asserted half a century ago by Rumford, 

 becomes, through the discovery of its mechanical equivalent, a 

 certainty. 



The form of force denoted by the name "heat" is plainly not 

 single, but includes several distinct, though mutually equivalent, 

 objects, three principal forms of which are distinguished in com- 

 mon lauguage : namely, I. Radiant Heat; II. Free (sensible) 

 Heat, Specific Heat ; and III. Latent Heat. 



There can be no doubt that radiant heat must be regarded as 

 a phenomenon of motion, especially since the recent detection 

 of phenomena of interference in the radiation of heat. But 

 whether there really exists, as is commonly assumed, a peculiar 

 rcther, of which the vibratory motion is perceived by us as radiant 

 heat, or whether the scat of this motion is the particles of mate- 

 rial bodies, is a question that is not yet made out. 



Still greater obscurity hangs about the essential nature of 

 specific heat, or what goes on in the interior of a heated body. 

 Not only does the unanswered question of the aether enter again 

 here, but, before we can be in a position to form any clear ideas 

 on this subject, we require to have an exact knowledge of the 

 internal constitution of matter. We are, however, still far from 

 having reached this point; for, in particular, we do not know 

 whether such things as atoms exist — that is, whether matter 



