54 CORRELATION OF PHYSICAL FORCES. 



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 dilating the air beneath it, and B to raise another 

 piston 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 tem- 

 perature ; 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 mer- 

 cury 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 contradictory 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 appliance 

 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 converted into a surplus 

 of heat, and be a creation of force ; and a deficit cannot be 

 supposed, for that would be annihilation offeree. I cannot, 

 however, see how the theoretical conception could be verified 

 by experiment ; the enormous weights and the complex 

 mechanical contrivances requisite to give the measure of power 

 yielded by matter in its less dilatable forms, would be far 

 beyond our present experimental resources. It would also 

 be difficult to prevent the interference of molecular forces, 

 the overcoming of which expends a part of the mechanical 

 power generated, but which could hardly be made to appear 

 in the result. We could not, for instance, practically realise 

 the above conception by the construction of a machine which 

 should act by the expansion and contraction of a bar of 



