THE CENTURY'S PROGRESS IN PHYSICS 



the great doctrine of conservation must now claim our 

 attention. 



As to Karl Friedrich Mohr, it may be said that his 

 statement of the doctrine preceded that of any of his 

 fellows, yet that otherwise it was perhaps least impor- 

 tant. In 1837 this thoughtful German had grasped the 

 main truth, and given it expression in an article pub- 

 lished in the Zeitschrift fur Physik, etc. But the article 

 attracted no attention whatever, even from Mohr's own 

 countrymen. Still, Mohr's title to rank as one who 

 independently conceived the great truth, and perhaps 

 first conceived it before any other man in the world 

 saw it as clearly, even though he did not demonstrate 

 its validity, is not to be disputed. 



It was just five years later, in 1842, that Dr. Julius 

 Robert Mayer, practising physician in the little German 

 town of Heilbronn, published a paper in Liebig's Annalen 

 on "The Forces of Inorganic Nature," in which not 

 merely the mechanical theory of heat, but the entire 

 doctrine of the conservation of energy, is explicitly if 

 briefly stated. Two years earlier Dr. Mayer, while 

 surgeon to a Dutch India vessel cruising in the tropics, 

 had observed that the venous blood of a patient seemed 

 redder than venous blood usually is observed to be in 

 temperate climates. He pondered over this seemingly 

 insignificant fact, and at last reached the conclusion 

 that the cause must be the lesser amount of oxidation 

 required to keep up the body temperature in the tropics. 

 Led by this reflection to consider the body as a machine 

 dependent on outside forces for its capacity to act, he 

 passed on into a novel realm of thought, which brought 

 him at last to independent discovery of the mechanical 

 theory of heat, and to the first full and comprehensive 



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