NEW SOURCES OF LIGHT. 3*9 



plate, and fourthly, rays causing air to become a conductor of elec- 

 tricity. The history of these discoveries can be briefly given. 



Kontgen's discovery of the rays that pass through metals and solids 

 opaque to light was made in 1895, and in the following year, Becquerel, 

 a distinguished French academician, discovered that salts of the metal 

 uranium (substances that had long been used in coloring china and 

 glass) emit invisible radiations capable of discharging electrified bodies 

 and of producing skiagraphic images on sensitive plates; he found that 

 potassio-uranic sulfate emits rays that pass through black paper and 

 give photographic impressions in the same way as Eontgen rays. This 

 property is not limited to the brilliantly fluorescent uranic salts, but is 

 shared by the non-fluorescent uranous salts, and is exhibited by com- 

 pounds whether phosphorescent or not, whether crystalline, melted or 

 in solution, as well as by the metal itself. The permanence of this 

 activity is amazing, substances kept in a double leaden box more than 

 three years continuing to exert the power. 



Shortly after the announcement by Becquerel, experimenters found 

 that other substances have the power of emitting 'Becquerel Kays,' 

 such as calcium and zinc sulfids and compounds of thorium. In 1898 

 Mine. Sklodowska Curie, working in the laboratory of the Municipal 

 School of Industrial Physics and Chemistry in Paris, devised a special 

 apparatus for measuring the electrical conductivity of the air when 

 liiider the influence of 'radio-active bodies,' and by its means studied 

 the behavior of pitchblende (uraninite), and of other uranium minerals; 

 iinding that some specimens of pitchblende had three times as much 

 energy as uranium itself, she came to the conclusion that the peculiar 

 property is due to some unknown body contained in the minerals and 

 not to uranium. Examining the mineral with the aid of her husband, 

 the two found a substance analogous to bismuth, four thousand times 

 stronger than uranium, which was named 'Polonium,' in honor of the 

 native land of Mme. Curie. In Pecehfber of the same year, the lady 

 received the Gegner prize of 4,000 francs awarded by the Academy of 

 Sciences, as a substantial appreciation of her discovery, and later in the 

 same month Mme. and M. Curie announced that they had found a 

 second body in pitchblende, which they named 'Eadium.' More re- 

 cently, M. Debierne, working under the auspices of Mme. Curie, has 

 discovered a third body, which he calls 'Actinium,' an unfortunate 

 appellation because 'actinium' has already been used for an element 

 announced by Dr. Phipson and since discarded. 



These three 'radio-active' substances do not possess identical prop- 

 erties; their rays are unequally absorbed and are differently affected in 

 a magnetic field; moreover radium emits visible rays, while polonium 

 does not. Nor have they the same chemical affinities; polonium be- 

 longs to the bismuth group, radium to the barium and actinium to the 



