THE BECQUEREL RAYS 133 



knowledge of the phenomena of radiation, and furnishes 

 as conspicuous an example as the Herschels of the her- 

 edity of a scientific faculty. Antoine Charles, born in 

 1788, was famous for his electric researches; Edmond, his 

 son, born in 1820, was author, with his father, of a treatise 

 on Electricity and Magnetism, and investigated the phe- 

 nomenon of phosphorescence, of which more anon ; and 

 Henri Becquerel, born in 1852, while engaged in extend- 

 ing his father's work, made the wonderful discovery of 

 the emission of rays from certain minerals containing 

 the rare metals uranium and thorium. We must first, 

 however, consider Edmond Becquerel's work. 



Certain substances, after illumination, do not at once 

 cease to give back light, but continue to glow, even after 

 the source of light has been removed. Such substances, 

 one of the best known of which is fluor-spar, are called 

 phosphorescent. Some years ago, an attempt was made to 

 utilise one of such substances a carbonate of lime con- 

 taining traces of sulphide of manganese under the name 

 of ' luminous paint.' Another class of substances has the 

 property of transforming the vibrations they receive when 

 illuminated into vibrations of a different period; among 

 these are extract of horse-chestnut, and many artificial 

 colouring-matters, one of the most striking being the 

 lovely pink dye, eosin. Such bodies do not continue to 

 emit light after excitation ; they were differentiated from 

 the former by Edmond Becquerel, and are termed fluor- 

 escent. The tendency of these substances is to convert 

 short wave-lengths into longer ones. Thus a solution of 

 acid sulphate of quinine, which is perfectly colourless by 

 transmitted light, is opaque to ultra-violet rays. It reflects 

 them, and at the same time increases the wave-length ; 

 hence by reflected light the solution appears to possess a 

 violet shimmer. 



It was in 1838 that Faraday investigated the luminous 



