41 2 Spectrum of the Spontaneous Radiation of Radium. [Oct. 29, 



of the experiment, so far as concerns helium, was negative ; which must 

 not of course be interpreted as excluding the presence of helium, but 

 only as showing that, if present, the conditions are not favourable to 

 the appearance of its spectrum.* 



On the first photograph that was taken, the two strongest lines of 

 the nitrogen band spectrum were faintly seen, but a photograph with 

 a new screen and a longer exposure showed no trace of the nitrogen 

 bands. In the first case it might be, that some very minute particles 

 of radium bromide had attached themselves to the screen, and by their 

 independent glow had given rise to the lines of nitrogen which were 

 on the photographic plate. 



About 1 centigramme of French radium bromide, which was in the 

 form of small particles, was put into a very small glass tube scarcely 

 larger than was necessary to contain it. The tube was securely closed 

 and left for 2 months. As the a rays, being unable to escape, would 

 probably occupy the interstices between the radium bromide particles, it 

 seemed desirable to examine whether as helium, or still in some 

 precedent condition, they would show their presence in the glow 

 spectrum. The tube was exposed, immediately in front of the slit, for 

 168 hours. The spectrum shows a strong continuous spectrum from 

 the fluorescence of the glass, and faintly the bands of nitrogen, but no 

 other lines with certainty. We intend to photograph again the spectrum 

 of the glow from this tube, after a longer time has passed for an 

 accumulation of the a rays, and of the gas-like emanation. 



When the radium bromide is covered with a plate of quartz, the 

 continuous spectrum, due to the fluorescence of the quartz, is not 

 only strong, but extends a long way into the ultra-violet. It can be 

 traced on the photograph as far as 2500. 



After a few hours the quartz darkens under the action of the radium 

 bromide, the brown stain extending through the complete substance 

 of a plate one-tenth of an inch in thickness. The stain is due probably 

 to the reduction of silicon. 



Experiments were made in the hope of throwing light upon the 

 shift found in the photograph of the radium glow spectrum, repro- 

 duced on the plate of our first paper. As subsequent photographs of 

 this spectrum were entirely free from any trace of shift, the shift 

 found on the first plate must have been accidental. Repeated photo- 



* M. Henri Becquerel has quite recently investigated the scintillation observed 

 on a phosphorescent screen when excited by radium. He comes to the conclusion 



(1) " Ce sont les rayons a qui provoquent la phosphorescence scintillante " ; 



(2) "Ces faits etablissent sinon une demonstration, du moins une grande pr6- 

 sornption en favour de 1'hypothese qui attribuerait la scintillation a des clivages 

 provoques irregulierement sur 1'ecran cristallin par 1'action continue plus ou 

 moins prolongee des rayons a." (' Coinptes Rendus, 1 vol. 137, pp. 633, 634 ; 

 October 27, 1903.) 



