§ =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. The Discovery of Radium. § =87.= Mme. Curie[102] determined the radioactivity of many uranium and thorium compounds, and found that there was a proportion between the radioactivity of such compounds and the quantity of uranium or thorium in them, with the remarkable exception of certain natural ores, which had a radioactivity much in excess of the normal, and, indeed, in certain cases, much greater than pure uranium. In order to throw some light on this matter, Mme. Curie prepared one of these ores by a chemical process and found that it possessed a normal radioactivity. The only logical conclusion to be drawn from these facts was that the ores in question must contain some unknown, highly radioactive substance, and the Curies were able, after very considerable labour, to extract from pitchblende (the ore with the greatest radioactivity) minute quantities of the salts of two new elements--which they named “Polonium” and “Radium” respectively--both of which were extremely radioactive. [102] See Madame SKLODOWSKA CURIE’S _Radio-active Substances_ (2nd ed., 1904). M. Debierne has obtained a third radioactive substance from pitchblende, which he has called “Actinium.” Chemical Properties of Radium.