PHYSICS OF NINETEENTH CENTURY 179 



eternal and indestructible ; the amount of energy in 

 the Universe became, constant and immutable, in all 

 conditions and for all ages. The principles passed 

 from safe guides for empirical advance in knowledge, 

 into philosophic dogmas of doubtful validity. 



When heat is transformed into work, or work into 

 heat, the equivalence between them is completely 

 expressed by Joule's results. But, although it is 

 always possible to transform the whole of a given 

 quantity of work into heat, it is not generally possible 

 to perform completely the reverse change. In steam 

 engines, and other heat engines, it is always found 

 that only a fraction of the heat supplied is transformed 

 into mechanical energy ; the remainder, which passes 

 from hotter to colder parts of the system, does not 

 become available for the performance of useful work. 

 Every heat engine needs a difference of temperature, 

 a hot body or source of heat and a cold body or con- 

 denser. The possible efficiency of the engine increases 

 as this difference of temperature increases, and, in a 

 theoretically perfect frictionless engine, the efficiency 

 is independent of the form of the engine or the nature 

 of the working substance, and gives a clue to an 

 absolute scale of temperature which also is independent 

 of the properties of any one substance. 



On these lines the principle of the relations between 

 heat and work, the science of thermodynamics, was 

 established, chiefly by the labours of Sadi Carnot 

 (1796-1832), William Thomson, afterwards Lord 

 Kelvin (1824-1907), and R. J. E. Clausius (1822-1888). 



The consequences of thermodynamic reasoning 

 have not only enabled the engineer to place on a firm 

 footing the theory of the heat engine, but have aided 



