126 SCIENTIFIC THOUGHT. 



that confusion which the indefinite use of the word had 

 caused, especially among Continental writers. One of 

 the first practical applications of this idea as referred 

 to the motive power of heat in Carnot's sense was made 

 23. by William and James Thomson in 1849. Thev had 



Application 



07 William both fully realised that lowering of temperature might 



and James * 



Thomson. ^ accompanied by the doing of work by heat, and 

 that elevation of heat to a higher temperature meant 

 expense of work. If, therefore, work could be done by 

 heat without lowering the temperature, there was an 

 apparent gain of motive power without corresponding 

 expenditure. It was known that water at freezing 

 temperature expanded in becoming ice : it was capable 

 of doing work, frequently very destructive work, with- 

 out a lowering of temperature. In order to convert 

 water into ice of the same temperature, heat must be 

 abstracted. Here, then, was a case of a possible trans- 

 ference of heat without fall of temperature, and the 

 creation or gam of great power to do work; but, ac- 

 cording to Carnot's principle, equality of temperature 

 implied an absence of expenditure of work. So here 

 was a case of gain without expenditure of power sim- 

 ply by a transference of heat at freezing-point. James 

 Thomson 1 saw the solution of the paradox. If water 



1 The reasoning of James Thorn- j sion to which he had been led by 

 son, based again upon the impossi- '. reasoning on principles similar to 

 bility of a perpetual motion, is given those developed by Carnot with ref- 

 in the following passage of his com- erence to the motive power of heat. 

 munication to the Royal Society of It was that water at the freezing- 

 Edinburgh, dated January 2, 1849 point may be converted into ice by 

 (reprinted in his brother, Lord Kel- \ a process solely mechanical, and yet 

 vin's, 'Math, and Phys. Papers,' without the final expenditure of any 



vol. i. p. 156) : " Some time ago my 

 brother, Prof. William Thomson, 



mechanical work. This at first ap- 

 peared to me to involve an impossi- 



pointed out to me a curious conclu- bility, because water expands while 



