144 PROGRESS OF SCIENCE IN THE CENTURY. 



Davy's Contribution. A more delicate experiment 

 was devised in IT 9 9 by Sir Humphry Davy, who ar- 

 ranged a clockwork for rubbing two pieces of ice 

 against one another in the vacuum of an air-pump, 

 and observed that part of the ice was melted, although 

 the temperature of the receiver was kept below the 

 freezing point. From this he concluded somewhat 

 diffidently that friction causes vibration of the par- 

 ticles, which is heat ; a conclusion which he strength- 

 ened in 1812 in the statement that " the immediate 

 cause of the phenomenon of heat is motion and the 

 laws of its communication are precisely the same as 

 the laws of the communication of motion." Thomas 

 Young was another of the early supporters of Count 

 Rumford's view. 



Work of Carnot. Meanwhile important progress 

 was made, by Dulong and Petit (1815), Haugergues 

 (1822), and others, on the measurement of temper- 

 atures by means of thermometers; by Faraday and 

 others on the liquefaction of gases, and on many other 

 subjects associated with heat : but the next important 

 step in general theory was made by Sadi Carnot 

 (1796-1832), who, in 1824, published his estimate 

 of the amount of work that can be got from a steam- 

 engine, and introduced the fruitful idea of a revers- 

 ible cycle of operations. But this was hardly known 

 until Sir William Thomson called attention to it in 

 1848. 



" Without this work of Carnot's, the modern theory 

 of energy, and especially the dynamical theory of 

 heat, could never have attained in so few years its 

 now enormous development." * 



" The two grand things which Carnot introduced, 

 which were entirely originated by him, and which left 

 * Prof . P. G. Tait's Recent Advances (1876), p. 95. 



