ON THE PHYSICAL VIEW OF NATURE. 129 



expressed in the first law, and the doctrine of the avail- 

 ability of energy as expressed in ihe second law. U was 

 Thomson who first clearly saw that the axitjni of the 

 impossibility of a perpetual motion would l)e infringed if 

 the first law of thermo-dynamics — the indestructibility of 

 energy — was accepted without the second. P'or practical 

 use, for doing work, it is not sufficient that energy be not 

 lost ; it must be available — get-at-aljle. Energy may Ije 

 in a condition in w'hich it is useless — hidden away — and 

 to liring it forth again may either be for us impossible 

 (if it be dissipated), or may require an expenditure of 

 work — i.e., of energy — to do so. The second law puts 

 into mathematical language another very inqxjrtant and 

 very striking property of the processes in nature. Let 

 us dwell on this a moment. 



The doctrine of the preservation of energy, of the 

 equivalence of the different forms of energy, tended to 

 put all the forms of energy on the same level. If they 

 be convertible, they appear to be of the same value. 

 If in doing work, energy was not consumed but only 

 changed, it stood to reason that it might be changed 

 back again, so that tlie work could be d(jne over again. 

 In other words, if all processes are purely mechanical 

 processes — modes of motion — a supposition which very 

 early forced itself with more or less clearness on the 

 pioneers of the science of energy, they must be reversible : 

 it must 1)0 possible to turn them round again, to undo 

 what has been done, or to do what has been undone. 

 Now the common-sense view of nature tells us at once 

 that this is impossible ; but it does not seem to have 

 struck the earlier propounders of the doctrine of the 



VOL. IL I 



