474 TEE POPULAR SCIENCE MONTHLY . 



interest as the development of a science within a short time and by a 

 small number of men, from the condition of a vague anticipation of 

 nature to that of a science with secure foundations, clear definitions and 

 exact boundaries."^^ Its development falls conveniently into three 

 stages : ( 1 ) The derivation of the two laws governing thermal trans- 

 formations of energy by Carnot, Eankine, Mayer, Joule, Clausius and 

 Kelvin. (3) The deductive application of the second law to all phys- 

 ico-chemical phenomena by Gibbs. (3) The application of probabilities 

 and statistical methods to the kinetic theory of gases by Clausius, 

 Kelvin, Maxwell and Boltzmann and the final derivation of the 

 theorems and equations of thermodynamics by statistical induction 

 from the average behavior of mechanical systems by Gibbs. 



" It must not be thought that heat generates motion or motion heat 

 (though in some respects this is true) but the very essence of heat or 

 the substantial self of heat is motion and nothing else.^^ In this 

 sentence from the Novum Organum it is clear that Bacon, like 

 Descartes, Count Eumford, Sir Humphry Davy and Young, had a 

 more or less definite notion of the dynamic nature of heat and its 

 convertibility into work. But the exact science which treats of heat as 

 a mode of energy begins with the publication, in^ 1884, of the " Eeflex- 

 ions sur la puissance motrice du feu" of Sadi Carnot, whom Lord 

 Kelvin calls the " prof oundest thinker in thermodynamic philosophy " 

 in the first half of his century.^^ In this little work we have the first 

 treatment of the heat engine as a reversible " cycle of operations," a 

 mechanism which can be worked backward with its every action 

 reversed ; and such a system is now known everywhere as a " Carnot 

 cycle." Carnot compared the motor power of heat to a fall of water.^® 

 As the power of the waterfall depends upon its height and the quantity 

 of fluid employed, so the motor power of heat depends, not upon the 

 nature of the working substance, but upon the quantity of heat em- 

 ployed and the difference in temperature between its source (the boiler) 

 and the sink (or exhaust cylinder) to which it flows. A heat motor, 

 then, requires a hot body and a cold body; the ideally perfect engine 

 would be completely reversible and the efficiency of engines working 

 between the same limits of temperature is the same. In other words, 

 heat can not perform work except by spontaneous flow from a higher to 

 a lower temperature. This is Carnot's principle, from which is derived 



^"lUd., 1877-8, XVII., 257. 



*' Bacon, " Novum Organum," English translation of 1850, p. 165. 



"Kelvin, "Popular Lectures," London, 1894, Vol. II., 460. 



" " On pent comparer la puissance motrice de la chaleur a celle d'une chute 

 d'eau: ... la puissance motrice d'une chute d'eau depend de la hauteur et de 

 la quantity du liquide; la puissance motrice de la chaleur depend aussi de la 

 quantity de calorique employ^, et de ce que nous appellerons le hauteur de sa 

 chute, c'est a dire de la difference de temperature des corps entre lesquels se 

 fait I'echange du calorique." Carnot, " Reflexions," 1824, 15. 



