1903.] of the Spontaneous Radiation of Radium. 411 



As the radium glow consists of light from nitrogen molecules stimu- 

 lated into luminosity by the presence of the more active radium 

 molecules, it was reasonable to suppose that the bromine molecules, 

 chemically associated with the latter, might also be sufficiently stimulated 

 to reveal their presence by the lines in the spectrum peculiar to them. 

 Photographs were accordingly taken of the poles of a vacuum tube 

 containing traces of atmospheric air together with bromine vapour. 

 The band spectrum of nitrogen appeared alone upon the plates when no 

 capacity was introduced ; but with the intercalation of a jar, the lines 

 of bromine came out in the photographs, in addition to the lines of air. 

 The experiment was then repeated at atmospheric pressure by enclosing 

 platinum electrodes in a glass bulb communicating with the atmosphere 

 by a narrow tube. Photographs of the coil discharge taken between 

 them revealed the ordinary band spectrum of nitrogen. A few drops 

 of bromine were then introduced into the bulb, filling it with bromine 

 vapour. Photographs were again taken of the discharge in the air now 

 heavily laden with bromine, but the spectrum remained precisely the 

 same as before the bromine was introduced, namely, that of nitrogen 

 only. 



We find in this experiment possibly a sufficient reason for the absence 

 of any of the lines of bromine in the glow spectrum : it may be that 

 stimulation from the active radium molecules affects preferentially the 

 nitrogen molecule, so that this molecule can be shaken into luminosity 

 by a stimulation which is insufficient to excite the bromine molecule to 

 a comparable extent. 



The experiment then suggested itself whether under similar con- 

 ditions of discharge, radium itself, when placed upon the electrodes, 

 would be able to show its presence by its characteristic lines in the 

 spectrum of the discharge taken between them. The result was nega- 

 tive, as in the case of bromine, no lines other than those of nitrogen 

 appearing upon the plate. A small jar was then put into the circuit 

 and another photograph taken, when the complete spectrum of radium 

 came out strongly, but without the band spectrum of nitrogen. 



If, as suggested by Rutherford, the a rays are connected with helium, 

 the experiment seemed worth making of taking a photograph of the 

 spectrum arising from their bombardment upon a zinc sulphide screen. 

 It seemed possible, though not very probable, that the encounters of 

 these bodies, at the enormous speed at which they travel, with the 

 molecules of air, and their final collision with the screen, might on that 

 hypothesis give rise to some of the radiations peculiar to helium and so 

 produce its spectrum on the plate. Fortunately the strong continuous 

 spectrum due to the fluorescence of the screen ends abruptly in the 

 violet a little before the place, at 3889, of the strongest line of helium in 

 the photographic region, and so leaves the spectrum quite free for the 

 detection of this line, even if it were only faintly present. The result 



