ADDRESS. 



Ill 



[peri- 

 and 



leveu 



full 



un- 



idily 



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

 think that the ahnost 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 ol' tlie first, as to assert that ecjuivalents of heat and work aro 

 not of ecpial value. While work can always be convei-ted into heat, heat 

 can only be converted into work under certain limitations. For every 

 pi-actical purpose the work is worth the most, and when wo speak of 

 equivalents, Ave use the word in the same sort of special sense as that iu 

 which chemists speak of ecjuivalents of golil and iron. The second law 

 teaches us that the real value of. heat, as a source of mechanical power, 

 depends upon the tomperatui'c of the body iu which it resides ; the hotter 

 the body iu relation to its sui'roundings, the more available the heat. 



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

 second law of Thormo-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 ineflicicncy of the steam-engine on the basis 

 of a comparison of the work actually got out of tho engine with the 

 mechanical equivalent of the heat supplied to tho boiler. Such calcula- 

 tions took into account only the first law of Thermo-dynamics, which deals 

 Avith the equivalents of heat nnd 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 laAv the fraction of the total 

 energy which can be converted into Avork 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 Avhich, according to the first law of 

 Thermo-dynamics, is resident in the coal. On a sounder vicAv of the 

 matter, the efttciency of tho steam-engine is found to be so high, that 

 there is no great margin remaining for improvement. The higher 

 initial temper-'iture possible in the gas-engine opens out much Avider 

 jwssibililies, and many good judges look forAvard to a time when the 

 steam-engine Avill have to giA'e Avay to its younger rival. 



To return to the theoretical question, we may say Avith 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 Avho has grasped this principle can fail to recognise its immense im- 

 portance in the system of the Universe. Eveiy 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 Avish 

 to inquire whether or not a proposed transfonnation 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- 



