A HISTORY OF SCIENCE 



published in the Zeitschrtft 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 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 dis- 

 puted. 



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

 Robert Mayer, practising physician in the little Ger- 

 man 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 ex- 

 plicitly 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 reach- 

 ed 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 inde- 

 pendent discovery of the mechanical theory of heat, 

 and to the first full and comprehensive appreciation 

 of the great law of conservation. Blood-letting, the 

 modern physician holds, was a practice of very doubt- 

 ful benefit, as a rule, to the subject ; but once, at least, 

 it led to marvellous results. No straw is so small that 



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