Energy Distribution in Spectra. 53 



and a guard-ring was made by scraping away the silver 

 in the planes through the dotted lines P P at right angles to 

 the axis o£ the tube. The silver deposit was also removed to 

 leave a rectangular window 20 x 4 mm. in the side of the 

 tube. A side tube S containing a mixture of rubidium 

 chloride and calcium was then fused on, and the whole 

 apparatus was exhausted by a Gaede pump. During ex- 

 haustion the apparatus was heated by a Bunsen burner to 

 remove films or gas from the walls. 



S was then raised to a cherry-red heat, whereupon the 

 rubidium chloride and calcium reacted, setting free the 

 rubidium *, which distilled over and formed a coating on 

 the walls of the cell. This coating was driven off the window 

 and the narrow strips of surface on each side of the guard- 

 ring by local heating with a small pointed gas-flame. S was 

 then sealed off at the constriction and helium was admitted 

 to the cell at a pressure of a few millimetres, so that a 

 luminous discharge could be made to pass between the 

 cathode and the anode (the former being in metallic con- 

 nexion with the inner wall of the tube) under a pressure 

 of 200 volts. The cell was then finally sealed off at the 

 constriction N. 



In order to increase the sensitiveness by obtaining the 

 colloidal modification of rubidium, the narrow end of the cell 

 was raised to a white heat in a blowpipe-flame, to allow a 

 small quantity of hydrogen to diffuse into the cell. On 

 passing a discharge the hydrogen rapidly combined with the 

 rubidium. This process was repeated till the metallic lining, 

 which was previously silvery white, became appreciably blue. 

 In this way, as Elster and Geitel discovered, the sensitiveness 

 in the selective region is greatly increased. 



The photo-electric currents given by this cell per unit 

 amount of energy falling on the incidence slit are shown in 

 fig. 10. It will be seen that the selective region of wave- 

 lengths to which rubidium is most sensitive covers the visible 

 spectrum, but the addition of potassium and csesium would 

 probably diminish the rapid drop which occurs towards the 

 blue and red wave-lengths. 



The advantages of quartz as compared with glass in the 

 construction of the cell are: (1) observations can be taken in 

 the ultra-violet, (2) the mixture from whicli rubidium is to 

 be generated can be heated in a side tube without risk of 

 collapse, (3) in the preliminary heating of the cell there 



* This method of reducing alkali metals from their chlorides was first 

 described by J. Hackspill. Ccmptes Rendus, cxli. p. 106 (1905). 



