Septembek 13, 1912] 



SCIENCE 



327 



meters.) The origin of the apparent dis- 

 crepancy between theory and experiment 

 lay in the tacit assumption that the quan- 

 tity of caloric in a Irilocalorie was the same 

 at different temperatures. There were no 

 experiments at that time available to dem- 

 onstrate that the caloric measure of heat as 

 work per degree fall, implied in Carnot's 

 principle, or more explicitly stated in his 

 equation, was not the same as the ealori- 

 metric measure obtained by mixing sub- 

 stances at different temperatures. Even 

 when the energy principle was established 

 its exponents failed to perceive exactly 

 where the discrepancy between the two 

 theories lay. In reality both were correct, 

 if fairly interpreted in accordance with 

 experiment, but they depended on different 

 methods of measuring a quantity of heat, 

 which, so far from being inconsistent, were 

 mutually complementary. 



The same misconception, in a more subtle 

 and insidious form, is still prevalent in 

 such common phrases as the following: 

 "We now know that heat is a form of 

 energy and not a material fluid." The 

 expeiiniental fact underlying this state- 

 ment is that our ordinary methods of 

 measuring quantities of heat in reality 

 measure quantities of thermal energy. 

 "When two substances at different tempera- 

 tures are mixed, the quantity remaining 

 constant, provided that due allowance is 

 made for external work done and for ex- 

 ternal loss of heat, is the total quantity of 

 energy. Heat is a form of energy merely 

 because the thing we measure and call heat 

 is really a quantity of energy. Apart from 

 considerations of practical convenience, we 

 might equally well have agreed to measure 

 a quantity of heat in accordance with Car- 

 not's principle, by the external work done 

 in a cycle per degree fall. Heat would 

 then not be a form of energy, but would 

 possess all the properties postulated for 



caloric. The caloric measure of heat fol- 

 lows directly from Carnot's principle, just 

 as the energy measure follows from the law 

 of conservation of energy. But the term 

 heat has become so closely associated with 

 the energy measure that it is necessary to 

 employ a different term, caloric, to denote 

 the simple measure of a quantity of heat as 

 opposed to a quantity of heat energy. The 

 measurement of heat as caloric is precisely 

 analogous to the measure of electricity as 

 a quantity of electric fluid. In the case of 

 electricity, the quantity measure is more 

 familiar than the energy measure, because 

 it is generally simpler to measure electricity 

 by its chemical and magnetic effects as a 

 quantity of fluid than as a quantity of 

 energy. The units for which we pay by 

 electric meter, however, are units of energy, 

 because the energy supplied is the chief 

 factor in determining the cost of produc- 

 tion, although the actual quantity of fluid 

 supplied has a good deal to do with the 

 cost of distribution. Both methods of 

 measurement are just as important in the 

 theory of heat, and it seems a great pity 

 that the natural measure of heat quantity 

 is obscured in the elementary stages of ex- 

 position by regarding heat simply as so 

 much energy. The inadequacy of such 

 treatment makes itself severely felt in the 

 later stages. 



Since Carnot's principle was adopted 

 without material modification into the me- 

 chanical theory of heat, it was inevitable 

 that Carnot's caloric, and his solution for 

 the work done in a finite cycle, should 

 sooner or later be rediscovered. Caloric 

 reappeared first as the "thermo-dynamic 

 function" of Rankine, and as the "equiva- 

 lence value of a transformation" in the 

 equations of Clausius ; but it was regarded 

 rather as the quotient of heat energy by 

 temperature than as possessing any special 

 physical significance. At a later date, 



