THE CENTURY'S PROGRESS IN PHYSICS 



chemical action. Thus he linked together light, chemi- 

 cal affinity, magnetism, and electricity. And, moreover, 

 he knew full well that no one of these can be produced 

 in indefinite supply from another. Nowhere, he says, 

 "is there a pure creation or production of power with- 

 out a corresponding exhaustion of something to supply 

 it." 



When Faraday wrote those words in 1840 he was 

 treading on the very heels of a greater generalization 

 than any which he actually formulated ; nay, he had it 

 fairly within his reach. He saw a great truth without 

 fully realizing its import ; it was left for others, ap- 

 proaching the same truth along another path, to point 

 out its full significance. 



in 



The great generalization which Faraday so narrowly 

 missed is the truth which since then has become familiar 

 as the doctrine of the conservation of energy the law 

 that in transforming energy from one condition to an- 

 other we can never secure more than an equivalent 

 quantity; that, in short, "to create or annihilate ener- 

 gy is as impossible as to create or annihilate matter; 

 and that all the phenomena of the material universe 

 consist in transformations of energy alone." Some phi- 

 losophers think this the greatest generalization ever 

 conceived by the mind of man. Be that as it' may, it is 

 surely one of the great intellectual landmarks of our 

 century. It stands apart, so stupendous and so far- 

 reaching in its implications that the generation which 

 first saw the law developed could little appreciate it ; 

 only now, through the vista of half a century, do we 

 begin to see it in its true proportions, 

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