HEAT. 69 



it contracts neighbouring substances the conclusion ap- 

 pears to me inevitable that the mechanical power produced 

 by heat will be definite, or the same for a given amount and 

 intensity of heat, whatever be the substance acted on. 



Thus, let A be a definite source of heat, say a pound of 

 mercury at the temperature of 400 ; let B be another equal 

 and similar source of heat : suppose A be employed to raise 

 a piston by the dilatation of air, and B to raise another pis- 

 ton by the dilatation of the vapour of water. Imagine the 

 pistons attached to a beam, so that they oppose each other's 

 action, and thus represent a sort of calorific balance. If A 

 being applied to air could conquer B, which is applied to 

 water, it would depress or throw back the piston of the latter, 

 and, by compressing the vapour, occasion an increase of 

 temperature ; this, in its turn, would raise the temperature 

 of the source of heat, so that we should have the anomaly 

 that a pound of mercury at 400 could heat another pound 

 of mercury at 400 to 401, or to some point higher than its 

 original temperature, and this without any adventitious aid : 

 it will be obvious that this is impossible, at least contradic- 

 tory to the whole range of our experience. 



The above experiment is ideal, and stated for the object 

 of giving a more precise form to the reasoning ; to bring the 

 idea more prominently into relief, all statements as to quan- 

 tities, specific heats, &c., so as to yield comparative results 

 for given materials, are omitted. The argument may be 

 thus stated in another form, viz. that by no mechanical appli- 

 ance or difference of material acted on can a given source 

 of heat be made to produce more heat than it originally 

 possessed ; and that, if all be converted into mechanical 

 power, an excess cannot be supposed, for that could be con- 

 verted into a surplus of heat, and be a creation of force ; and 

 a deficit cannot be supposed, for that would be annihilation 

 of force. I cannot, however, see how the theoretical concep- 

 tion could be verified by experiment ; the enormous weights 



