Since these record-making experiments by Joule, the matter has been
verified over and over again in all sorts of ways; and almost every kind
of display of energy has been measured with more or less exactness. Even
the amount of food oxidized in the human body is now known to be capable
of correlation with the other forms of energy, though necessarily very
minute exactness of measurement is scarcely attainable in this case. But
no scientist of to-day doubts that all the physiological processes of
animals or of plants conform exactly to the law of the conservation of
energy that energy is neither created nor destroyed by any means known
to science. In other words, the amount of energy in our world, if
science can at all determine such a matter, seems to be _a fixed
quantity_, gradually being dissipated into space, it is true, but
momently replenished from the sun at exactly the same rate now as
hundreds or thousands of years ago. And while this energy is in our
world it is always capable of exact correlation in all of its
multitudinous forms, and is transformable back and forth without
increase and without loss.
On the discovery of the radioactive substances in 1896, some persons
hastily concluded that the law of the conservation of energy was
contradicted by the astonishing way in which these substances acted.
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