114 



THE NEW KNOWLEDGE. 



phoresce. The photographs were taken from above. They 

 were caused by the induced light of induced radio-activity. 

 We have used the word " induced " here as descriptive of 

 this secondary or acquired radio-activity because it was 

 at first Professor Curie's opinion, as it was nearly every- 

 body's else, that bodies in the neighbourhood acquired 

 this temporary and secondary radio-activity because of the 

 mere presence of the radium, in the same way that iron 

 filings become magnetic in the presence of a magnet or as 

 the induced electric currents arise in an induction coil. 

 Some facts, however, soon became apparent, that have al- 

 tered completely this conception of the nature of the ac- 

 quired radio-activity. For example, Professor and Ma- 

 dame Curie performed the following experiment : Two 

 small flasks were connected together by a lateral tube fused 

 into their necks (Fig. 37), and having a stop-cock in the 



Fig. 37. 



tube joining them; the cock being closed, a solution of the 

 radio-active substance was passed into one of the flasks, 

 while a gelatinous white precipitate of sulphide of zinc, 



