THE SMALLEST PARTICLES OF MATTER 11 



tube strike a metal target within the tube, an intensely pene- 

 trating form of radiant energy emerges. This is known as x-rays 

 or Roentgen rays; they cause fluorescence in many substances, 

 and affect photographic plates. I well remember the thrill we 

 students got when, soon after Roentgen's discovery, Dr. L. H. 

 Friedburg showed us a photograph of the bones of his hand which 

 he had taken by means of x-rays emitted by one of the Crookes 

 tubes in the laboratory of Professor R. Ogden Doremus at the 

 College of the City of New York. 



Becquerel and Radioactivity 



Important consequences followed quickly upon Roentgen's dis- 

 covery. Professor Henri Antoine Becquerel of Paris (Nobel prize, 

 1903) had developed an interest in fluorescence and phosphores- 

 cence from his father, also a professor at Paris. In 1880, he had 

 prepared crystals of the double sulfate of potassium and uranium, 

 which glow beautfully when exposed to light. To see what con- 

 nection there might be between phosphorescence and x-rays, Bec- 

 querel wrapped a number of phosphorescent substances in ample 

 folds of black paper and placed them on photographic plate 

 holders. No effect was registered. Recalling his uranium salt, he 

 tried some that had been illuminated, and obtained a noticeable 

 photographic effect, which also appeared when he interposed a 

 thin glass plate to stop possible vapors. At first Becquerel thought 

 that this was due to phosphorescence; but later he found that the 

 photographic image was just as marked when the uranium salt 

 had been kept for weeks in a dark drawer. He also noted that, if 

 uranium salts are placed near a charged gold-leaf electroscope, 

 the uranium rays ionize the air and increase its electrical conduc- 

 tivity. As a result, the charge leaks away more rapidly and the 

 strips of gold leaf quickly fall together. Becquerel thus discov- 

 ered the phenomenon of radioactivity, and this had prompt and 

 extremely important repercussions in many fields of science. 



The Curies and Radium 



Marie Skladovska Curie (Nobel prize, 1903 and 1911), while 

 working for her Doctor's degree in Paris, began measuring with a 

 gold-leaf electroscope the relative radioactivity of various uranium 

 salts. She quickly found that the radioactivity is proportional to 

 the amount of uranium present, irrespective of its mode of chem- 

 ical combination. This indicated that radioactivity is property 

 of atoms. Thorium, too, was later found to be radioactive. For- 



