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SCIENCE 



[N. S. Vol. XXXVI. No. 924 



be almost exclusively adopted for this pur- 

 pose; but in many cases the caloric theory 

 would have the advantage of emphasizing 

 at the outset the importance of fundamen- 

 tal facts which are too often obscured in 

 the prevailing method of treatment. 



The explanation of the development of 

 heat by friction was one of the earliest 

 difficulties encountered by the caloric 

 theory. One explanation, maintained by 

 Cavendish and others, was simply that ca- 

 loric was generated de novo by friction in 

 much the same way as electricity. Another 

 explanation, more commonly adopted, was 

 that the fragments of solid, abraded in 

 such operations as boring cannon, had a 

 smaller capacity for heat than the original 

 material. Caloric already existing in the 

 substance was regarded as being squeezed 

 or ground out of it without any fresh ca- 

 lorie being actually generated. The prob- 

 ability of the second explanation was neg- 

 atived by the celebrated experiments of 

 Rumford and Davy; who concluded that 

 friction did not diminish the capacities of 

 bodies for heat, and that it could not be a 

 material substance because the supply ob- 

 tainable by friction appeared to be inex- 

 haustible. Rumford also showed that no 

 increase of weight in a body when heated 

 could be detected by the most delicate 

 apparatus available in his time. Calorie 

 evidently did not possess to any marked 

 extent the properties of an ordinary pon- 

 derable fluid ; but, if it had any real exist- 

 ence and was not merely a convenient 

 mathematical fiction, it must be something 

 of the same nature as the electric fluids, 

 which had already played so useful a part 

 in the description of phenomena, although 

 their actual existence as physical entities 

 had not then been demonstrated. Heat, as 

 Rumford and Davy maintained, might be 

 merely a mode of motion or a vibration of 

 the ultimate particles of matter, but the 



idea in this form was too vague to serve as 

 a basis of measurement or calculation. The 

 simple conception of caloric, as a meas- 

 urable quantity of something, sufficed for 

 many purposes, and led in the hands of 

 Laplace and others to correct results for 

 the ratio of the specific heats, the adiabatic 

 equation of gases, and other fundamental 

 points of theory, though many problems in 

 the relations of heat and work remained 

 obscure. 



The greatest contribution of the caloric 

 theory to thermodynamics was the produc- 

 tion of Carnot's immortal "Reflections on 

 the Motive Power of Heat." It is one of 

 the most remarkable illustrations of the 

 undeserved discredit into which the caloric 

 theory has fallen, that this work, the very 

 foundation of modern thermodynamics, 

 shovild still be misrepresented, and its logic 

 assailed, on the ground that much of the 

 reasoning is expressed in the language of 

 the caloric theory. In justice to Carnot, 

 even at the risk of wearying you with an 

 oft-told tale, I can not refrain from taking 

 this opportunity of reviewing the essential 

 points of his reasoning, because it affords 

 incidentally the best introduction to the 

 conception of caloric, and explains how a 

 quantity of caloric is to be measured. 



At the time when Carnot wrote, the in- 

 dustrial importance of the steam engine 

 was already established, and the economy 

 gained by expansive working was gener- 

 ally appreciated. The air-engine, and a 

 primitive form of the internal-combustion 

 engine, had recently been invented. On 

 account of the high value of the latent heat 

 of steam, it was confidently expected that 

 more work might be obtained from a given 

 quantity of heat or fuel by employing 

 some other working substance, such as 

 alcohol or ether, in place of steam. Carnot 

 set himself to investigate the conditions 

 under which motive-power was obtainable 



