for a screen exposed to Röntgen rays. Giesel made a screen of platino-cyanide of radio-active barium. The screen, very luminous at first, gradually turned brown in colour, and at the same time the crystals became dichroic. In this condition the luminosity was much less, although the active substance had increased in activity after preparation. Many of the substances which are luminous under the rays from active substances lose this property to a large extent at low temperatures[1].
116. Luminosity of radium compounds. All radium
compounds are spontaneously luminous. This luminosity is especially
brilliant in the dry haloid salts, and persists for long
intervals of time. In damp air the salts lose a large amount of
their luminosity, but they recover it on drying. With very active
radium chloride, the Curies have observed that the light changes
in colour and intensity with time. The original luminosity is
recovered if the salt is dissolved and dried. Many inactive preparations
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.
117. Spectrum of the phosphorescent light of radium and actinium. 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 self-luminous. A spectroscopic examination of the
slight phosphorescent light of pure radium bromide has been
made by Sir William and Lady Huggins[2]. 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
