PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 257 



where s indicates the quantity of the current, I tlic total resistance of tlie 

 circuit, and Ic a constant factor. The quantity of heat produced, liowever, is, 

 under like conditions, always proportional to the consumption of zinc in the 

 Lattery, (local action being of coui'se disregarded.) Were the conducting wire, 

 for instance, so much lengthened that the whole resistance of tlie circuit would 

 be doubled, and-thus raised to 21, the quantity of the current and consumption of 

 zinc would be thereby reduced to half, but tlie heat produced by the cuiTent 

 would be 



Thus with the quantity of current and consumption of zinc, the production of heat 

 also would be reduced to half. 



Quite otherwise is the result when the diminution of the quantity of the current 

 is ])roduced, not by the augmentation of the resistance to conduction, but by the 

 expenditure of powei". 



In a pn.'vious section of this work it has been seen that the strength of the 

 current, which traverses any electro-magnetic motor in a state of repose, is instantly 

 re(hiccd when the motor begins to rotate, and that the current becomes weaker 

 as the rotation is more rapid. Let us suppose that the burden of the machine be 

 so regulated that the strength of the current of the rotating machine be just 

 half as great as in that at rest, then, with the quantity of current reduced to half, 

 the consumption of zinc will also be reduced to half j but the production of heat 

 will have decreased in a quite different proportion. Since now the strength of 

 the current is ^ s, but the resistance the same as in the machine at rest, namely, 

 I, we shall have as the quantity of heat produced 



Thus the zinc consumption reduced to half produces only a quantity of heat 

 reduced to one-fourth ; a part of the zinc-consumpUon, therefore, is not employ edin 

 the production of heat, hut in the performance of mechaniccd worJc, or, in other 

 words, for the quantity of heat ^ w, an equivalent mechanical work has been 

 performed. 



We observe a similar state of things if we investigate the performance of labor 

 by human or animal forces. Animal heat, we know, is generated by a slow com- 

 bustion, kept up through the process of breathing. For the oxygen which we 

 inspire, carbonic ac^id and vajxjr are exhaled ; with every breath, therefore, a 

 dehnite quantity of carbon and hydrogen leaves the body, and the corporeal 

 mass must necessanly undergo a corresponding diminution ; a diminution which, 

 if iKjt determinable by weight foi' every breath, is readily so for an interval of a 

 few hours. This loss of material ;n the process of breathing is replaced through 

 the recej)tion of food. 



But the proportiim between the producticjn of heat and the consumption of 

 corporeal matter is quite diirerent, according as the person remains perfectly at 

 rest, or is engaged' in the performance of some more or less considerable labor. 

 The production of heat and consiniq)tion of oxygen, and, consequently, the bodily 

 diminution of weight, are at a minimum, if the individual continues for some 

 time sitting or lying in complete inactivity. If he perform, on the contrary, some 

 sticmious labor, both the consumj^tion of oxygen and the reduction of weight 

 will be foimd in the same space of time to have been much more sensible. 

 Through the accelerated breathing and more rapid ])ulsation the ])roduction oi 

 heat in the b(jdy is undoubted!}' augmented, l)ut it results from the j)rincii)les of 

 the mechanical theory of heat that the development of warmth cannot bo taken 

 as directly proportional to the consumption of oxygen, but that the increased 

 interchange of matter in the l)ody serves only in i)art for the jiroduction of heat, 

 while the rest has been spent in the producing mechanical eilect. 



The coiTcctness of this proposition has been verified by Uirn in a series of 

 17 S 



