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. But
further and more accurate experiments have set this matter at rest, as
indeed might have been expected; for the law of gravitation itself is
not more immovably established in the make-up of the universe than this
magnificent law that energy cannot be created by any means which we call
natural.

In all ages there have been men who have spent their lives in the vain
effort to invent a machine out of which work could constantly be
obtained without the expenditure upon it of an equal amount of work. But
the United States patent office has got so tired of receiving
applications for patents based on this idea of perpetual motion that
they have long since refused to issue any such patent where this
principle is the manifest object; and I suppose the governments of other
countries have taken a similar stand. And why? Because they know that
energy cannot now be created by any device, no matter how ingenious; and