THE MOTIVE POWER OF HEAT. 123 



whom he lived were entirely ignorant of the fund of knowledge he 

 had accumulated. His brother, who was called upon to read the manu- 

 script of his memoir on the motive power of heat in order to see that 

 it was clear enough to be understood by others than scientists, says 

 that he never did understand why Carnot made this one exception. 

 It seems that his solitary life in small garrisons, in the office and in 

 the laboratory served to increase his natural reserve. Yet he was not 

 in the least reticent in a small company; he took part willingly in the 

 gayest joys and abandoned himself to the liveliest conversation. His 

 language was then full of witticism, biting but not malignant, original 

 but not eccentric, sometimes paradoxical, but never with any other 

 pretension than that of an active mind. 



It was in 1824 while still an officer on the general staff that Carnot 

 published his 'Keflections on the Motive Power of Heat.' Struck with 

 the fact that chance alone seemed to direct the construction of steam 

 engines, he undertook to raise to the rank of a science the art that was 

 still so imperfect in spite of its importance. He investigated the 

 phenomena of the production of motion by heat from the most general 

 point of view, independent of any particular mechanism and of any 

 particular agent. It was only some years after his death that the 

 value of his work was revealed to his fellow countrymen by an echo 

 from England. However, it did merit the attention of a few French 

 scientists, notably the celebrated engineer, Clapeyron, who in 1834 pub- 

 lished in the Journal Ecole Polyteclwique a paper which was a com- 

 ment upon and an extension of the ideas of Carnot, in which he called 

 attention to Carnot 's reasoning, represented Carnot 's processes in an 

 analytical form and arrived at some new results, usefully applying, 

 and for the first time, the principle of Watt's indicator diagram to 

 the geometrical exhibition of the different quantities involved in the 

 cycle of operations by which work is derived from heat by the tem- 

 porary changes it produces in the volume and molecular state of bodies. 

 It was through this work of Clapeyron that Carnot 's ideas became 

 known to Lord Kelvin, who presented them to the world in 1848, 

 pointing out that they enabled us to give for the first time an absolute 

 definition of temperature, i. e., a thermodynamic scale of temperature 

 which is independent of the properties of any particular substance. 

 On this scale the absolute values of two temperatures are defined to 

 be in the same ratio as the amounts of heat-energy taken in and 

 rejected by a perfect (i. e., reversible) thermodynamic engine, working 

 with its source and its refrigerator at the higher and lower of these 

 temperatures respectively. Lord Kelvin showed that the ratio between 

 these quantities of heat-energy depends only on these two working 

 temperatures and is independent of the substance used in the engine, 



