THE CENTURY'S PROGRESS IN PHYSICS 



tive men could accept the new point of view ; older phi- 

 losophers, their minds channelled by preconceptions, 

 could not get into the new groove. So strikingly true 

 is this in the particular case now before us that it is 

 worth while to note the ages at the time of the revolu- 

 tionary experiments of the men whose work has been 

 mentioned as entering into the scheme of evolution of 

 the idea that energy is merely a manifestation of matter 

 in motion. Such a list will tell the story better than a 

 volume of commentary. 



Observe, then, that Davy made his epochal experi- 

 ment of melting ice by friction when he was a youth of 

 twenty. Young was no older when he made his first 

 communication to the Koyal Society, and was in his 

 twenty -seventh year when he first actively espoused the 

 undulatory theory. Fresnel was twenty-six when he 

 made his first important discoveries in the same 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 jears older when he 

 formulated his own doctrine of dissipation of energy. 

 And Clausius and Rankine, who are usually mentioned 

 P 225 



