ON THE PHYSICAL VIEW OF NATURE. 167 

 The train of thought methodically and comprehen- 44. 



Willard 



sively followed out in Gibbs's various memoirs had its 

 origin in the early speculations of William Thomson 

 (Lord Kelvin) and Clausius, to which I referred above. 

 Thomson was the first who, in adopting (after much 

 hesitation) the mechanical view of the phenomena of heat, 

 the doctrine of the convertibility and equivalence of the 

 different forms of energy, recognised that, in order to 

 describe natural phenomena correctly, this view required 

 a qualification. The change of the different forms of 

 energy into each other can for the most part take place 

 only in one direction; there is a general tendency in 

 nature towards a degradation or dissipation of energy. 

 Energy, though not lost, becomes less useful, less avail- 

 able. The least available form of energy is heat ; and 

 it is in that form that in all natural changes a por- 

 tion of energy becomes lost, dissipated, or hidden away. 

 Thus we have to recognise the difference between 

 available and unavailable, between useful and useless, 

 energy. In the sequel Thomson showed in definite 

 instances l how to calculate the available and the un- 



1 See 'Math, and Phys. Papers,' j in this connection, is applied by him 

 vol. L No. LIX., 1852, " On a Uni- j to the negative of the idea we most 



versal Tendency in Nature to the naturally wish to express. It would 



Dissipation of Mechanical Energy" ; only confuse the student if we were 



and No. LXIIL, 1853, "On the to endeavour to invent another 



Restoration of Mechanical Energy term for our purpose." He then 



from an unequally heated Space. proceeds to use the term entropy 



In Tait's ' Sketch of Thermodynam- in an altered sense, in which it 



ics' (1868), we read (p. 100): "It measures the available instead of 



is very desirable to have a word to the unavailable energy, creating 



express the availability for work of for some time a great confusion 



the heat in a given magazine, a and some unnecessary irritation, 



term for that possession the waste See on this the early editions of 



of which is called Dissipation. Un- Clerk Maxwell's excellent ' Theory 



fortunately the excellent word en- of Heat,' and the footnote to p. 



tropy, which Clausius has introduced 189, 8th ed., and Clausius, 'Die 



