Physicists know that certain metals used in experimental work become radio-active while under the direct influence of the x-rays. It has been asserted, however, that such activity ceases with the removal of such influence or shortly thereafter. The fact that emanations proceed from radium or radium-salts, by which substances exposed to them become radio-active, is well known.
The physical similarity between the rays emitted by the x-ray tube and the Becquerel rays, at least the similarity between the x-rays and the beta rays emitted from radium, their penetrating power, their phosphorescent action, their action on photographic plates in black paper and plate-holders, their action in causing air to become conductive for electricity—all these phenomena made the existence also of a similarity between the nature of these rays seem plausible to us, and therefore we began experiments by which we found that radio-activity could not only be induced
SCHILLER H, O'DONNELL PS. RADIO-ACTIVITY INDUCED BY THE X-RAYA PRELIMINARY REPORT. JAMA. 1910;55(17):1462–1463. doi:10.1001/jama.1910.04330170042014
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