98 Rayleigh's Address to the British Association. 



For the second law so far contradicts the usual statement of the 

 first, as to assert that equivalents of heat and work are not of 

 equal value. While work can always be converted into heat, heat 

 can only be converted into work under certain limitations. For 

 every practical purpose the work is worth the most, and when we 

 speak of equivalents, we used the word in the same sort of special 

 sense as that in which chemists speak of equivalents of gold and 

 iron. The second law teaches us that the real value of heat, as a 

 source of mechanical power, depends upon the temperature of the 

 body in which it resides ; the hotter the body in relation to its 

 surroundings, the more available the heat. 



In order to see the relations which obtain between the first and 

 the second law of Thermo-dynamics, it is only necessary for us to 

 glance at the theory of the steam-engine. Not many years ago 

 calculations were plentiful, demonstrating the inefficiency of the 

 steam-engine on the basis of a comparison of the work actually 

 got out of the engine with the mechanical equivalent of the heat 

 supplied to the boiler. Such calculations took into account only 

 the first law of Thermo-dynamics, which deals with the equivalents 

 of heat and work, and have very little bearing upon the practical 

 question of efficiency, which requires us to have regard also to the 

 second law. According to that law the fraction of the total energy 

 which can be converted into work depends upon the relative 

 temperatures of the boiler and condenser, and it is, therefore, 

 manifest that, as the temperature of the boiler cannot be raised 

 indefinitely, it is impossible to utilize all the energy which, 

 according to the first law of Thermo-dynamics, is resident in the 

 coal. On a sounder view of the matter, the efficiency of the 

 steam-engine is found to be so high that there is no great margin 

 remaining for improvement. The higher initial temperature 

 possible in the gas-engine opens out much wider possibilities, and 

 many good judges look forward to a time when the steam-engine 

 will have to give way to its younger rival. 



To return to the theoretical question, we may say with Sir W. 

 Thompson, that, though energy cannot be destroyed, it ever tends 

 to be dissipated, or to pass from more available to less available 

 forms No one who has grasped this principle can fail to recog- 

 nize its immense importance in the system of the Universe. 

 Every change — chemical, thermal, or mechanical — which takes 



