190 PHYSICAL SCIENCE 



Evidence of further changes is also forth- 

 coming. Surfaces exposed to the emanation of 

 radium retain a small part of their residual 

 activity for several years without appreciable 

 diminution. By taking advantage of differences 

 in volatility and other properties, Rutherford has 

 traced three more stages in the transmutation of 

 the radium products. The first is half accom- 

 plished in about sixteen years, and involves /8 and 

 7 rays ; the second takes six days, and is also 

 accompanied by the emission of ^ and 7 rays ; 

 while the third, marked by a radiation, needs 

 136 days to sink to half its initial activity. 

 Rutherford calls the deposited radio-active matter 

 radium A, B, etc., and writes these eight genera- 

 tions of the radium pedigree as : Radium, radium 

 emanation, radium A, radium B, radium C, 

 radium D, radium E, radium F. 



Radium F has been shown by Rutherford to 

 be identical with the substance separated by 

 Madame Curie from pitch-blende and called by 

 her polonium. Could it be prepared pure, it 

 should be several hundred times as active as 

 radium, but, as half of it would vanish in about 

 143 days, the labour and expense needed for its 

 separation would afford but a short-lived specimen 

 for the investigator. 



A somewhat similar series of changes has been 

 made out in the case of thorium. Another radio- 

 active constituent too has been separated from 

 pitch-blende and named actinium by Debierne. 

 It is derived indirectly from uranium and forms 

 an X product, an emanation, and several solid 

 deposits distinguished as actinium A, B, C, and D. 



The quantities of matter involved in any radio- 



