RADIOACTIVITY— ARTIFICIAL AND NATURAL 293 



duce the air-pressure by yet another ten-fold, the splendor will fade out, 

 but now the glass of the tube itself, in the region opposite the negative 

 electrode, will be shining with a pale green glow. This too is a beauti- 

 ful sight, but decidedly not one to be enjoyed for a long time at close 

 range, for it is attended by invisible rays very dangerous to health 

 and even life. These are the X-rays. 



Many people have heard that Roentgen discovered the X-rays 

 because he kept some photographic plates in a box which happened to 

 lie near the tube, and found one day that the plates were fogged. It 

 is true that he did observe the fogging of plates by the rays; it is true 

 also that for years afterward, people were telling in various parts of 

 Western Europe and America how they too had noticed that their 

 plates were spoiled when left by accident in the neighborhood of dis- 

 charge-tubes, but unluckily they had only felt annoyed and had 

 resolved to keep their next batches of plates in safer places. However 

 it was something else that led Roentgen to the discovery. He had in 

 his laboratory some sheets of phosphorescent substances; and he 

 found that whenever one of them was in the neighborhood of the tube 

 it would shine; and what was more, it would shine even when hidden 

 from the tube by a sheet of black cardboard. The X-rays (as he presently 

 named them) were able to pierce substances opaque to light, and to 

 cause phosphorescent substances to shine. Roentgen very soon 

 discovered other properties of the rays, but these were the earliest. 

 Moreover the greenish glowing of the glass in the X-ray tube itself 

 resembles the light of phosphorescence; so, here were two apparent 

 connections between penetrating X-rays on the one hand, and phos- 

 phorescence - on the other. 



Now we come to radioactivity. There was a man in Paris whose 

 attention was caught by these facts, and his name was not Curie. 

 Curie is the second great name in the story of radioactivity; the first is 

 Becquerel. I put down his first name (Henri) as well, for Becquerel — 

 like Curie and Bernoulli and Darwin and others — is the name of a 

 dynasty of scientists, of which Henri was the third. Henri Becquerel 

 seems to have reasoned ^ after this fashion: "X-rays and phosphores- 

 cence are found together; therefore, wherever there is phosphorescence, 

 there may be rays like X-rays." So he took photographic plates and 

 wrapped them in dark paper, and then he took one phosphorescent 

 substance after another and (after making them luminous by exposing 

 them to light) laid them in succession beside the plates, and after an 

 interval looked to see whether there had been fogging. Time after 



^ Or fluorescence: a careful distinction is drawn between phosphorescence and 

 fluorescence by specialists, but need not detain us here. 



' The idea is, however, ascribed to Henri Poincare by Marie Curie. 



