of a crystal already coloured may be intensified. Now it is well known that if a coloured crystal of fluorspar is heated it is thereby discoloured, and the effect of subjecting a crystal which has been so treated to the action of the rays from radium is of interest.

There is a particular variety of fluorspar known as "blue-john," which occurs in Derbyshire, coloured so deeply violet that a crystal of any considerable size appears almost black. If such a crystal is strongly but carefully heated, it is deprived of its colour and becomes perfectly transparent. On now exposing the crystal to the action of the rays from radium, the crystal becomes again coloured, but there is a remarkable fact about the experiment, namely, that the crystal does not assume its original violet colour but is transformed into a crystal of sky-blue tint. At the same time the crystal has become strongly fluorescent, and may be seen in a dark room by the light which it emits. This fluorescence persists for a remarkably long time, the duration of which may be gathered from the following experiment. Four crystals of different samples of

removal from influence of radium.

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fluorspar were exposed to the ẞ and y radiation of a considerable quantity of radium for some days, after which they were removed and found to have been coloured by the radiation which had been falling upon them. The crystals were then placed upon a sensitive photographic plate at its four corners and left to stand for an hour in a dark room. As might have been expected, on developing, the plate was found to have been affected by the fluorescent crystals which had been placed upon it. This is shown in Fig. 21. The experiment was repeated at various intervals, and even after a fortnight three of the four crystals were able to affect the photographic plate when the latter was exposed to their action for an hour. The action of the fourth crystal had, however, by that time become too feeble to be detected by the photographic plate. The result obtained, ten days after removal from the radium, can be seen from Fig. 22. It will be noticed that although the intensity of the impression obtained was much less than immediately after removing the crystals from the action of the radium, the effect is still appreciable.

K

It might at first sight seem that the crystals had in some way acquired the property of emitting some radiation other than light after exposure to the radium. But if this is so, the rays emitted must be very easily absorbed, for by interposing a thin sheet of black paper between the crystals and the photographic plate the action can be completely stopped. On the other hand this is just what would be expected if the action were due to the light emitted by the crystals.

PHOSPHORESCENCE OF RADIUM COMPOUNDS

There is an interesting property of radium salts which is allied to those which have just been discussed. When a compound of radium is examined in a dark room it is found that it emits a light which can easily be seen from considerable distances if the room is sufficiently dark.

In this connection the somewhat remarkable observation was made by the Curies that radium which contained a certain amount of barium salt mixed with it was frequently more luminous than purer specimens of radium salts. Now, remembering the power of pro