ON I'UK PFIYSICAL VIKW OF NATl'RK. 1G7 



The train of lliuugliL nietliodiciilly and cuinpreheii- «. 



* 1 i* 11 1 '/"I'll* Wnl*rd 



sively toiloweil out in (fibbss various memoirs had it« o ''»»»• 

 origin in the early speculations of William Thomson 

 (Lord Kelvin) and C'lausius, to which 1 ivfcrrt'd 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. Tiie 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 seqviel Thomson showed in definite 

 instances ^ how to calculate the availal)le and tlie un- 



^ See ' Math, and Phys. Papers,' in this connection, is applied by liiin 



vol. i. No. LIX., 18.02, " On a Uni- 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 whicii 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 arai/aljilitij for work of for some time a great confusion 



the heat in a given magazine, a and some uiniccessarj' irritJition. 



term for that possession the waste See on this the early editic^ns of 



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



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



<ro/)?/, which C'lausius has introduced 189, 8th ed., and Clausiua, 'Die 



