i68 PHYSICAL SCIENCE 



activity as a test for the presence of those sub- 

 stances which possess it. A deHcate electroscope 

 will show easilya leak of electricity with a substance 

 having an activity of about the one-hundredth part 

 of that possessed by uranium. The activity of 

 pure radium has been estimated as about two 

 million times that of uranium ; and such radium is 

 a definite, well-marked chemical element, like other 

 elements, forming salts and other chemical com- 

 pounds, and giving strong bright lines when heated 

 and examined with a spectroscope. Spectrum 

 analysis has hitherto been the most delicate means 

 at our disposal for detecting the presence of the 

 chemical elements ; but in the preparation of 

 radium from pitch-blende its spectrum only began 

 to appear when, in the prolonged process of 

 fractionisation, the product had reached an activity 

 of about fifty times that of uranium. 



It appears from these figures that the electro- 

 scopic method of detecting radio-active matter is 

 several thousand times more sensitive than the 

 most refined methods of spectrum analysis, and in 

 other cases a still greater sensitiveness seems to 

 have been reached. History has again repeated 

 itself. When the spectroscope was first placed in 

 the hands of chemists, it revealed the existence of 

 several elements which occurred in quantities too 

 small to be detected by any other means then 

 known. In a similar way additional elements have 

 now been detected and isolated by the help of the 

 newer and more powerful method of research. 



In the year 1899 Professor Rutherford of 

 Montreal, now Sir Ernest Rutherford, Cavendish 

 Professor at Cambridge, discovered that the radia- 

 tion from uranium consists of two distinct parts. 



