be so closely associated, so mutually
convertible, with heat? All analogy seemed to urge the truth of
this inference; all experiment tended 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 demonstration. 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 admitted, reached it earlier than
either Joule or Colding, if neither brought it to quite so clear a
demonstration. The names of these three Germans are Mohr, Mayer,
and Helmholtz. Their share in establishing 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 important. In 1837 this thoughtful German had grasped
the main truth, and given it expression in an article published 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
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 i