i66 PHYSICAL SCIENCE 



Although the superficial similarity between 

 Becquerel rays and Rontgen rays has proved for 

 the most part misleading, the relations between 

 the two branches of the subject are so intimate 

 that it is impossible to study satisfactorily the 

 phenomena of radio-activity without a knowledge 

 of the results previously and simultaneously 

 reached by the investigation of electric discharge 

 through gases. 



After Becquerel's discovery of the photographic 

 and electric activity of uranium, it was found 

 that, like Rontgen rays, the rays from uranium 

 produced electric conductivity in air and other 

 gases through which they passed. Compounds 

 of thorium, too, were found to possess similar 

 properties. In the year 1900, M. and Mme. 

 Curie made a systematic search for these effects 

 in a great number of chemical elements and com- 

 pounds, and in many natural minerals. They 

 found that several minerals containing uranium 

 were more active than that metal itself. Pitch- 

 blende, for instance, a substance consisting chiefly 

 of an oxide of uranium, but containing also traces 

 of many other metals, was especially active. 

 When obtained from Cornwall its activity was 

 about equal to that of the same weight of uranium, 

 but Samples from the Austrian mines were found 

 to be three or four times as effective. The 

 presence of some more active constituent was thus 

 suggested. To examine this point, the various 

 components of pitch-blende were separated chemi- 

 cally from each other and their radio-activities 

 determined. In this way three different sub- 

 stances, radium, polonium, and actinium, all 



