u6 TEE POPULAR SCIENCE MONTHLY 



opaque substances, while casting shadows of others. Most wonderful 

 seemed the statement that by these rays the bones of the hand could be 

 seen. Seventeen years later these Eontgen rays are used in every hos- 

 pital, and reveal the inmost secrets of the body. But this is not their 

 interest to the physicist, but rather the fact that they have opened up a 

 whole field of facts previously unsuspected, so that an investigator ig- 

 noring them would to-day be held the greatest of old fogies. How did 

 Eontgen come to discover the X-rays? No doubt there was a certain 

 element of chance. We are told that he had covered the discharge tube, 

 the so-called Crookes tube, with black paper, so that no light should get 

 out from it, and that Eontgen's attention was attracted by the fluores- 

 cence, or faint shining with light, of a piece of paper lying on the table, 

 the paper being covered with the salt of barium platinocyanide. But 

 why did this piece of paper coated with this uncommon chemical happen 

 to be lying on the table, and why had Eontgen covered the Crookes 

 tube with black paper? "We find that barium platinocyanide was one 

 of the substances that had been investigated by previous investigators 

 as to its fluorescence, and that such paper was a commercial article in 

 Germany. Eontgen must then have suspected that there was some 

 property of the Crookes' tube that would cause fluorescence, so that the 

 presence of this fluorescent paper was not accidental at all. This is 

 then a striking example of what I have before stated. A further one is 

 given by a discovery made the next year in Paris. Eontgen's discovery 

 had set the world on fire, and had given rise to a renewed interest in the 

 subject of fluorescence. Noteworthy among fluorescent substances are 

 the salts of uranium, and these were examined by Henri Becquerel, the 

 third generation of physicists of that name. Becquerel placed uranium 

 salts against a photographic plate wrapped up in black paper, and soon 

 found that the plate was affected, even through the opaque paper. At 

 first Becquerel thought that the uranium had this property only after 

 being exposed to the sun's light, but he soon found that the same prop- 

 erties were possessed by uranium salts that had been formed in the 

 dark, and had never seen the sun. In short these salts are constantly 

 emitting a new sort of radiation, now known as Becquerel rays. Physi- 

 cists now began to look for other substances than uranium which had 

 these properties, with the result that it was found that uranium-bearing 

 ores were found to contain other substances having the properties in a 

 far higher degree, and at last the Curies were able to separate a new 

 element, which was named radium. The field of radioactivity thus 

 opened up has become an enormous one, and many substances have been 

 discovered having radioactive properties. Here is again an illustration 

 of the impossibility of distinguishing between physics and chemistry, 

 for although Mme. Curie is a chemist, the Nobel prize in chemistry was 

 awarded a few years ago to Professor Eutherford, professor of physics 



