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. 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 they refuse to become a party to any scheme that seems to imply that this modern creation of energy is within the bounds of possibility.