Contemporary Advances in Physics — XII. 

 Radioactivity 



By KARL K. DARROW 



IN the year 1896, which fell near the beginning of the great trans- 

 formation of modern physics, Henri Becquerel heard that Roentgen 

 had discovered strange rays proceeding from an electric discharge- 

 tube while the discharge was passing and the glass walls of the tube 

 were phosphorescing. Suspecting that the new rays were connected 

 with the phosphorescence, Becquerel tested samples of some of the 

 substances which naturally phosphoresce. It happened that one which 

 he tested was a compound of uranium. He wrapped the sample in 

 paper to shut in the light of its phosphorescence, and set it beside a 

 photographic plate; for the rays of Roentgen had disclosed themselves 

 by acting on such plates. Becquerel had made a happy guess; for 

 the compound affected the plate. Yet his original idea was altogether 

 wrong; for the effect had nothing to do with the phosphorescence of 

 the compound, it was due to the uranium itself and faithfully reap- 

 peared when other and non-phosphorescent compounds were used in- 

 stead, and even when a piece of the pure metal was set beside the plate. 

 It was an instance of a fallacious idea having guided a keen observer 

 to a great discovery — not the first in the history of physics, and as- 

 suredly not the last. 



Thereupon Pierre and Marie Curie, having verified that the effect of 

 any quantity of any compound of pure uranium is strictly proportional 

 to the amount of uranium in it, noticed that the effect of certain 

 natural rocks and minerals containing uranium was much greater 

 than that which their content of the metal should produce. Suspect- 

 ing that there was some constituent of the rocks having the same 

 property as the uranium but in a degree much greater, they set about 

 the task of getting the uranium and the inert matter out of the way 

 and isolating the more potent substance. It was a long task; to 

 speak of "winnowing" the pile of rock would be to use a comically 

 feeble metaphor, and as for the proverbial needle in a haystack, it 

 could have been extracted with incomparably less trouble than the 

 few hundredths of a gramme of the active substance which were latent 

 in the ton of raw material. Eventually the Curies did liberate it, or 

 rather them, for there were several active substances; and one of them 

 was named radium, and their strange property was called radioactivity. 

 This was the first of the words containing the magic sylkibles radio, 

 syllables which are one of the special symbols of our epoch ; were the 



55 



