THE MYSTERY OF MATTER. 



183 



of tlie vacuum-tube, clue to the impact of the cathode rays, was 

 a condition necessary to the production of the X rays. But it 

 was found later that when cathode rays impinged on a plate of 

 metal, more especially a heavy metal like platinum, although no 

 phosphorescence appeared, yet Eontgen rays of a specially 

 penetrating character were produced. This idea that phos- 

 phorescence was the cause of X rays, though erroneous, led 

 indirectly to discoveries of the greatest importance. To it was 

 due the epoch-making discovery of radio-activity and radium. 



Professor Henri Becquerel of Paris was experimenting with 

 various bodies which became phosphorescent after exposure to 

 sunlight in order to see if they emitted X rays, when it occui'red 

 to him to try a salt of the rare metal uranium. He took some 

 uranium salt, wrapped it well up in black paper, and placed it 

 on a prepared photographic plate, with a layer of metal foil 

 between the plate and the salt. He wrapped up the whole in 

 black paper and put it in a dark place for 24 hours. He 

 then examined the plate and found it "fogged" as if acted on by 

 light. He next tried some uranium salt prepared in the dark, 

 which had never been exposed to light at all, keeping it wrapped 

 up in the same way for 24 hours, and found the result to be the 

 same. This was evidently due to the formation and emission 

 by the uranium of penetrating rays under ordinary conditions. 

 These rays from uranium were found to be composed of streams 

 of corpuscles. They have been called Becquerel rays, from the 

 name of the discoverer. The property of emitting these rays is 

 called radio-activity, and the substances possessing it are said to 

 be radio-active. Crooks then showed that the radiation was 

 mainly due to a substance present in the uranium which he 

 called uranium X. Then M. Curie, whose untimely death last 

 year was so great a loss to science, and Madame Sklowdowski 

 Curie his talented wife, took up the subject and discovered 

 that the mineral pitch-blende, which is an oxide of uranium 

 mixed with various other metallic compounds, was several times 

 more radio-active than uranium itself. It was thus evident that 

 the pitch-blende contained small quantities of some other 

 substance much more active than uranium. By a very elaborate 

 process M. and Madame Curie managed to separate out two new 

 radio-active substances, one of which they named polonium, and 

 the other, much the more active of the two, they called radium. 



The radio-active elements, that is, those which are known to 

 emit corpuscular rays, are uranium, thorium, actinium, and 

 radium. Several others have been thought to occur, but either 



