Eddy.] 340 [-j^ne 16. 



energy. It thus appears that it is possible to avail ourseh*cs of the heat 

 existing in bodies below the lowest thermometric levels of surrounding 

 objects. 



It may be objected that the syren renders a perpetual motion a possi- 

 bility. That depends upon the definition of perpetual motion which we 

 adopt. In the popular acceptation of that term, the process of the syren, 

 as well as that of Maxwell, would make something near that possible. 

 But when correctly viewed, the process of the syren does not imply the 

 possibility of a perpetual motion, any more than does combustion or using 

 the available energy of any chemical process. 



It simply proposes to employ the finite amount of energy, existing in a 

 given body in the form of heat, in a given way. It is admitted by all. 

 that this heat could, a part of it, be made to do work by parting with some of 

 it to a cooler body. The question is, whether this last part which has been 

 imparted to the cooler body can be restored or transferred to the warmer 

 T)ody again without the expenditure of energy. Rankine evidently believed 

 such a transfer possible, for in a paper on the "Reconcentration of the 

 Mechanical Energy of the Universe,"* he has supposed it possible to re- 

 flect radiations in such a way as to give the universe such differences of 

 temperature as to ensure it a new lease of life. Clausius, in his admirable 

 paper on the "Concentrationof Rays of Light and Heat,"f has shown the 

 general impossibility of such a reconcentration as Rankine supposed, when 

 the radiating bodies are at rest ; nevertheless, no such impossibility may 

 finally appear in case of the actual universe, which is a sj^slem of moving 

 bodies. 



The law of the dissipation of energy has been applied to the universe at 

 large, and if the consequences which have been drawn from its supposed 

 validity are to be regarded as no longer expressing a necessary law, then 

 we are led to affirm that without change in the laws of nature as at 

 present known to us, it is possible for increasing diff"erences of tempera- 

 ture to be caused without the expenditure of energy, however improbable 

 the supposition may be that such is the fact, and however improbable it 

 may be that such differences-fire actually being caused on a scale sufficient 

 to interfere in any practical way with the progress of the dissipation of 

 energy as affirmed by Thomson, or check the increase of the entropy of 

 tlie universe as stated by Clausius. 



Still, it may be remarked, that a large part of the exchange of heat in 

 the universe takes place in the radiant form ; and it seems to me that it 

 remains to be proved what the fact actually is, and consequently I must 

 regard it as still an open question as to whether, on the whole, the available 

 energy of the universe is being dissipated and its entropy increased or not. 



Lest the foregoing remarks should be construed as in any sense under- 

 valuing the splendid discoveries of Clausius, Thomson and Rankine in the 



*Phil. Mag., Series Iv, Vol. Iv. 

 jMiTh. Th. ol Ifeiit, Cli.ijittM- vii. 



