ve] * ~ PROPERTIES OF THE RADIATIONS 169 
in colour and intensity with time. The original luminosity is 
recovered if the salt is dissolved and dried. Many inactive pre- 
parations of radiferous barium are strongly luminous. The writer 
has seen a preparation of impure radium bromide which gave out 
a light sufficient to read by in a dark room. The luminosity of 
radium persists over a wide range of temperature and is as bright 
at the temperature of liquid air as at ordinary temperatures. A 
slight luminosity is observed in a solution of radium, and if crystals 
are being formed in the solution, they can be clearly distinguished 
in the liquid by their greater luminosity. 
110. Spectrum of the phosphorescent light of radium. 
Compounds of radium, with a large admixture of barium, are 
usually strongly self-luminous. This luminosity decreases with 
Increasing purity, and pure radium bromide is only very feebly 
selfluminous. A spectroscopic examination of the slight phos- 
phorescent hight of pure radium bromide has been made by Sir 
William and Lady Huggins. On viewing the light with a direct 
vision spectroscope, there were faint indications of a variation of 
luminosity at different points along the spectrum. In order to get 
a photograph of the spectrum within a reasonable time, they made 
use of a quartz spectroscope of special design which had been 
previously employed in a spectroscopic examination of faint celestial 
objects. After three days’ exposure with a slit of 1/450 of an inch 
in width, a negative was obtained which showed a number of 
bright limes. The magnified spectrum is shown in Fig. 33. The 
lines of this spectrum were found to agree not only in position but 
also in relative intensity with the band spectrum of nitrogen. The 
band spectrum of nitrogen and also the spark spectrum? of radium 
are shown in the same figure. 
Some time afterwards Sir William Crookes and Prof. Dewar* 
showed that this spectrum of nitrogen was not obtaimed if the 
radium was contained in a highly exhausted tube. Thus it 
1 Proc. Roy. Soc. 72, pp. 196 and 409, 1903. 
2 The spark spectrum of the radium bromide showed the H and Kk lines of 
calcium and also faintly some of the strong lines of barium. The characteristic 
lines of radium of wave-lengths 3814:59, 3649-7, 4340°6 and 2708°6, as shown 
by Demarcay and others are clearly shown in the figure. The strong line of wave- 
length about 2814 is due to radium. 
> British Assoc. 1903. 
