262 



Mr. W. Crookes. 



[May 31, 



May 31, 1883. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The Bakerian Lecture— '» On Radiant Matter Spectroscopy. 

 A New Method of Spectrum Analysis," was delivered by 

 William Crookes.. JF.R.S. Received May 24, 1883. 



The following is an Abstract : — 



For several years I have been examining the phenomena presented 

 by various substances when struck by the molecular discharge from 

 the negative pole in a highly exhausted tube. I have ventured to 

 call this discharge "radiant matter," and under its influence a large 

 number of substances emit phosphorescent light, some faintly and 

 others with great intensity. On examining the emitted light in the 

 spectroscope most bodies give a faint continuous spectrum, with a 

 more or less decided concentration in one part of the spectrum, the 

 superficial colour of the phosphorescing substance being governed by 

 this preponderating emission in one or other part of the spectrum. 

 Sometimes, but more rarely, the spectrum of the phosphorescent 

 light is discontinuous, and it is to bodies manifesting this pheno- 

 menon that my attention has been specially directed. 



For a long time past I have been haunted by a bright citron- 

 coloured band or line appearing in these phosphorescent spectra, 

 sometimes as a sharp line, at others as a broader nebulous band, 

 but having always a characteristic appearance and occurring uni- 

 formly in the same spot. The best way to bring out the band is to 

 treat the substance under < examination with strong sulphuric acid, 

 drive off excess of -acid by' heat, and finally to raise the temperature 

 to redness. The anhydrou-s sulphate thus left frequently shows the 

 citron band in the radiant matter tube, when before this treatment the 

 original substance shows nothing. I soon came to the conclusion that 

 the substance I was in search of was an earth, but on attempting to 

 determine its chemical properties I was baffled. 



Much chemical evidence tended to support the view that the band 

 might be due to a compound of lime. By neglecting the portion 

 showing least citron band, and separating all the elements present 

 which gave little or none, I could generally concentrate the citron 

 band into a solution which —according to our present knowledge of 

 analytical chemistry — should contain little else than the earths, 



