THE STORY OF NINETEENTH-CENTURY SCIENCE 



measurable amount of heat, and no more. Joule found, 

 for example, that at the sea-level in Manchester a pound 

 weight falling through seven hundred and seventy-two 

 feet could generate enough heat to raise the temperature 

 of a pound of water one degree Fahrenheit. There was 

 nothing haphazard, nothing accidental, about this ; it 

 bore the stamp of unalterable law. And Joule himself 

 saw, what others in time were made to see, that this 

 truth is merely a particular case within a more general 

 law. If heat cannot be in any sense created, but only 

 made manifest as a transformation of another kind of 

 motion, then must not the same thing be true of all 

 those other forms of "force" light, electricity, magnet- 

 ism which had been shown to be so closely associated, 

 so mutually convertible, with heat? All analogy seemed 

 to urge the truth of this inference ; all experiment tend- 

 ed to confirm it. The law of the mechanical equivalent 

 of heat then became the main corner-stone of the greater 

 law of the conservation of energy. 



But while this citation is fresh in mind, we must turn 

 our attention with all haste to a country across the 

 Channel to Denmark, in short and learn that even as 

 Joule experimented with the transformation of heat, a 

 philosopher of Copenhagen, Colding by name, had hit 

 upon the same idea, and carried it far towards a demon- 

 stration. And then, without pausing, we must shift yet 

 again, this time to Germany, and consider the work of 

 three other men, who independently were on the track 

 of the same truth, and two of whom, it must be admit- 

 ted, reached it earlier than either Joule or Colding, 

 if neither brought it to quite so clear a demonstra- 

 tion. The names of these three Germans are Mohr, 

 Mayer, and Helmholtz. Their share in establishing 



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