294 BELL SYSTEM TECHNICAL JOURNAL 



time the result was completely negative, but at last he came upon a 

 chemical compound of the element uranium which was phosphorescent 

 and which fogged the plate. It is not recorded that he shouted 

 "Eureka!" but he had as good reason so to do as Archimedes. He 

 had discovered the first-to-be-known example of radioactivity. 



Now comes the strange and paradoxical part : Becquerel had arrived 

 at his great discovery by following a false clue. There is really no 

 connection whatever between radioactivity and phosphorescence, and 

 it was purely an accident that a compound which was phosphorescent 

 had happened to contain an element which was radioactive. In trying 

 to make a simile for what then happened, I have adopted the rather 

 frivolous comparison which follows. Suppose that you were to meet a 

 man who was wearing a blue serge suit, and notice that he was speaking 

 a foreign language — -Swedish, let us say. For some reason this would 

 interest you particularly, and you would decide to look for other 

 examples of people speaking Swedish. You would begin by reasoning 

 that "Swedish speech was associated with a blue serge suit, and there- 

 fore any man who is wearing blue serge may speak Swedish." You 

 would then listen to everyone whom you passed in the street who was 

 wearing blue serge, and the first few whom you heard would prove 

 to be speaking English. This would prevent you from believing that 

 a blue suit necessarily entails the speaking of Swedish; but you might 

 continue nevertheless, and eventually find another man who was 

 wearing blue serge and speaking Swedish. Now if you were like 

 some people, I am afraid you would send a communication to a scienti- 

 fic journal, announcing that it is a principle that everyone speaking 

 Swedish is wearing a blue serge suit. But not if you were like Bec- 

 querel ! If you were like Becquerel you would trail the man for weeks; 

 and sooner or later you would come upon him wearing a grey suit or 

 a brown one, and still he would be speaking Swedish. In the course 

 of time you would doubtless come upon other people who never wore 

 blue serge and yet spoke Swedish. Finally you would realize that it 

 was just a piece of luck that you had happened to discover a man 

 speaking Swedish, by making the fallacious assumption that all such 

 men wear blue. Now in this case of Becquerel's, the phosphorescence 

 was only a feature of the clothes which the uranium happened to be 

 wearing, or more literally, the chemical compound in which it was 

 involved. But the radioactivity was a feature of uranium itself, and 

 that is what Becquerel proceeded to prove; first by testing various 

 other chemical compounds of uranium which were not in the least 

 phosphorescent, and then by testing the pure uncompounded metal 

 itself. 



