X-rays and Becquerel rays.
§ 86. As is commonly known, what are called X-rays are produced when an electric discharge is passed through a high-vacuum tube. It has been shown that these rays are a series of irregular pulses in the ether, which are set up when the kathode particles strike the walls of the glass vacuum tube,[101] and it was found that more powerful effects can be produced by inserting a disc of platinum in the path of the kathode particles. It was M. Becquerel who first discovered that there are substances which naturally emit radiations similar to X-rays. He found that uranium compounds affected a photographic plate from which they were carefully screened, and he also showed that these uranium radiations, or “Becquerel rays,” resemble X-rays in other particulars. It was already known that certain substances fluoresce (emit light) in the dark after having been exposed to sunlight, and it was thought at first that the above phenomenon exhibited by uranium salts was of a like nature, since certain uranium salts are fluorescent; but M. Becquerel found that uranium salts which had never been exposed to sunlight were still capable of affecting a photographic plate, and that this remarkable property was possessed by all uranium salts, whether fluorescent or not. This phenomenon is known as “radioactivity,” and bodies which exhibit it are said to be “radioactive.” Schmidt found that thorium compounds possess a similar property, and Professor Rutherford showed that thorium compounds evolved also something resembling a gas. He called this an “emanation.”
[101] They must not be confused with the greenish-yellow phosphorescence which is also produced: the X-rays are invisible.