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URING the latter part of the nineteenth century a great deal of work was done upon electrical discharges in rarefied gases. In 1895 Röntgen made the epoch-making discovery that such a discharge was the source of very penetrating radiations. These radiations he called X-rays on account of their unknown nature, and he found that they possessed the power of making a gas a conductor of electricity by producing in it a great number of positively and negatively charged carriers or ions. Besides ionizing a gas, the X-rays were found to affect a photographic plate just as light rays do and to be able to penetrate thin sheets of the metals and many other bodies which are opaque to light. It was found in the course of experimentation that these X-rays were closely related to the stoppage of the cathode particles or corpuscles, and the phosphorescence on the walls of the vacuum tube which these corpuscles excite. In 1897 J. J. Thomson found that these cathode particles or corpuscles were small negatively charged particles of an apparent mass only one seven-hundredth that of the hydrogen atom and that in a "high vacuum" tube in a strong electric field they acquired a velocity approximating that of light. All the properties of the corpuscles were found to be the same, no matter what kind of gas or electrodes were in the discharge tube. Their mass was found to vary with their velocity in such a way that the whole mass of the corpuscle could be ascribed to the electric charge which it carried. From this most important discovery it was concluded that all the common substances were partly made up of corpuscles, and this conclusion has been strengthened by all later discoveries. After Thomson's discovery, Stokes showed that the sudden stoppage of the corpuscles by the walls of the discharge tube caused intense electromagnetic disturbances to travel out from the point of impact. These disturbances are the X-rays and travel with the velocity of light.

When Röntgen announced his discovery, it created a great impetus in the study of everything related to electrical discharges. Now it had been known for a long time that some bodies like the uranium salts phosphoresce when exposed to sunlight, and it occurred to H. Becquerel that such a phosphorescing body might emit X-rays, this