A HISTORY OF SCIENCE 



field; and Arago, who at once became his champion, 

 was then but two years his senior, though for a decade 

 he had been so famous that one involuntarily thinks of 

 him as belonging to an elder generation. 



Forbes was under thirty when he discovered the po- 

 larization of heat, which pointed the way to Mohr, then 

 thirty-one, to the mechanical equivalent. Joule was 

 twenty-two in 1840, when his great work was begun; 

 and Mayer, whose discoveries date from the same year, 

 was then twenty-six, which was also the age of Helm- 

 holtz when he published his independent discovery of 

 the same law. William Thomson was a youth just past 

 his majority when he came to the aid of Joule before 

 the British Society, and but seven years older when he 

 formulated his own doctrine of the dissipation of energy. 

 And Clausius and Rankine, who are usually mentioned 

 with Thomson as the great developers of thermo-dy- 

 namics, were both far advanced with their novel studies 

 before they were thirty. With such a list in mind, we 

 may well agree with the father of inductive science 

 that "the man who is young in years may be old in 

 hours." 



Yet we must not forget that the shield has a reverse 

 side. For was not the greatest of observing astrono- 

 mers, Herschel, past thirty-five before he ever saw a 

 telescope, and past fifty before he discovered the heat 

 rays of the spectrum ? And had not Faraday reached 

 middle life before he turned his attention especially to 

 electricity? Clearly, then, to make this phrase com- 

 plete, Bacon should have added that " the man who is 

 old in years may be young in imagination." Here, 

 however, even more appropriate than in the other case 



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