126 



SCIENTIFIC THOUGHT. 



23. 



Application 



^ 



Thomson. 



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 

 by William and James Thomson in 1849. They had 



* 



^oth ^ u ^7 realised that lowering of temperature might 

 fo e acc0 mpamed 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 gain 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 Thom- 

 son, based again upon the impossi- 

 bility of a perpetual motion, is given 

 in the following passage of his com- 

 munication to the Royal Society of 

 Edinburgh, dated January 2, 1849 

 (reprinted in his brother, Lord Kel- 

 vin's, ' Math, and Phys. Papers.' 

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

 my brother. Prof. William Thomson, 

 pointed out to me a curious conclu- 



sion to which he had been led by 

 reasoning on principles similar to 

 those developed by Carnot with ref- 

 erence to the motive power of heat. 

 It was that water at the freezing- 

 point may be converted into ice by 

 a process solely mechanical, and yet 

 without the final expenditure of any 

 mechanical work. This at first ap- 

 peared to me to involve an impossi- 

 bility, because water expands while 



