adow of any
substance interposed between the tube and the screen will appear upon
the phosphorescence plate.

The wonderful shadow pictures produced on the phosphorescence screen,
or the photographic plate, would seem to come from some peculiar form
of light, but the exact nature of these rays is still an open question.
Whether the Roentgen rays are really a form of light--that is, a form
of "electro-magnetic disturbance propagated through ether," is not fully
determined. Numerous experiments have been undertaken to determine this,
but as yet no proof has been found that the rays are a form of light,
although there appears to be nothing in their properties inconsistent
with their being so. For the moment most investigators are content to
admit that the term X-ray virtually begs the question as to the intimate
nature of the form of energy involved.

VIII. THE CONSERVATION OF ENERGY

As we have seen, it was in 1831 that Faraday opened up the field of
magneto-electricity. Reversing the experiments of his predecessors, who
had found that electric currents may generate magnetism, he showed that
magnets have power under certain circumstances to generate electricity;
he proved, indeed, the interconvertibility of electricity and magnetism.
Then he showed that all bodies are more or less subject to the influence
of magnetism, and that even light may be affected by magnetism as to its
phenomena of polarization. He satisfied himself completely of the
true identity of all the various forms of electricity, and of the
convertibility of electricity and chemical action. Thus he linked
together light, chemical affinity, magnetism, and electricity. And,
moreover, he knew full well that no one of these can be produced in
indefinite supply from another. "Nowhere," he says, "is there a pure
creation or production of power without a corresponding exhaustion of
something to supply it."

When Faraday wrote those words in 1840 he was treading on the very heels
of a greater generalization than any which he actually formulated; nay,
he had it fairly within his reach. He saw a great truth without fully
realizing its import; it was left for others, approaching the same truth
along another path, to point out its full significance.

The great generalization which Faraday so narrowly missed is the truth
which since then has become familiar as the doctrine of the conservation
of energy--the law that in transforming energy from one conditio