ADDRESS. 11 



to the demands that are made upon them by a growing science, and I 

 think that the almost unavoidable use of the word equivalent in the 

 statement of the first law is partly responsible for the little attention that 

 is given to the second. 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 iuto 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 use 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 l'elation 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 calcula- 

 tions 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 utilise 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 hi°-lier 

 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. 

 Thomson, 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 recognise its immense im- 

 portance in the system of the Universe. Every change — chemical, thermal, 

 or mechanical — which takes place, or can take place, in Nature, does so 

 at the cost of a certain amount of available energy. If, therefore, we wish 

 to inquire whether or not a proposed transformation can take place, the 

 question to be considered is whether its occurrence would involve dissipa- 

 tion of energy. If not, the transformation is (under the circumstances of 

 the case) absolutely excluded. Some years ago, in a lecture at the Royal 

 Institution, I endeavoured to draw the attention of chemists to the import- 



