and the Laws regarding the Nature of Heat. 3 



combined with an increase of elasticity, or a mechanical work_, 

 or a combination of both ; a mechanical work being the equiva- 

 lent for an increase of temperature. Heat can only be measured 

 by its effects; and of the two effects mentioned, mechanical 

 work is peculiarly applicable here, and shall therefore be chosen 

 as a standard in the following investigation. I name a unit of heat, 

 the quantity which, on being communicated to any gas, is able 

 to produce the quantity of work a; or to speak more definitely, 

 which is able to raise a kilogrammes to a height of one metre.'^ 

 Afterwards, at page 12, he determines the numerical value of the 

 constant a, according to the method of Meyer*, and obtains a 

 number which completely agrees with that obtained in a manner 

 totally different by Joule. In carrying out the theory, however, 

 that is, in developing the equations by means of which his con- 

 clusions are arrived at, he proceeds in a manner similar to Cla- 

 peyron, so that the assumption that the quantity of heat is con- 

 stant is still tacitly retained. 



The difference between both ways of regarding the subject has 

 been laid hold of with much greater clearness by W. Thomson, 

 who has applied the recent discoveries of Regnault on the tension 

 and latent heat of steam to the completing of the memoir of 

 Carnotf. Thomson mentions distinctly the obstacles which lie 

 in the way of an unconditional acceptance of Carnof s theory, 

 referring particularly to the investigations of Joule, and dwelling 

 on one principal objection to which the theory is liable. If it be 

 even granted that the production of work, where the body in 

 action remains in the same state after the production as before, 

 is in all cases accompanied by a transmission of heat from a warm 

 body to a cold one, it does not follow that by every such trans- 

 mission work is produced, for the heat may be carried over by 

 simple conduction ; and in all such cases, if the transmission 

 alone were the true equivalent of the work performed, an abso- 

 lute loss of mechanical force must take place in nature, which is 

 hardly conceivable. Notwithstanding this, however, he arrives 

 at the conclusion, that in the present state of science the prin- 

 ciple assumed by Carnot is the most probable foundation for an 

 investigation on the moving force of heat. He says, " If we 

 forsake this principle, we stumble immediately on innumerable 

 other difficulties, which, without further experimental investiga- 

 tions, and an entirely new erection of the theory of heat, are 

 altogether insurmountable.^' 



I believe, nevertheless, that we ought not to suffer ourselves 

 to be daunted by these difficulties ; but that, on the contrary, we 

 must look steadfastly into this theory which calls heat a motion, 

 as in this way alone can we arrive at the means of establishing 



* Ann. der Chim. und Pharm., vol. xlii. p. 239. 



t Transactions of the Royal Society of Edinburgh, vol. xvi. 



