natural to suppose that the atoms of the radio-active elements consist not only of the electrons in motion, but also of positively charged particles whose mass is about the same as that of the hydrogen or helium atom.
It will be shown later that only a minute fraction of the atoms of the radio-element need break up per second in order to account for the radiations even of an enormously active element like radium. The question of the possible causes which lead to this atomic disintegration and the consequences which follow from it will be discussed later in chapter XIII.
96. Experiments with a zinc sulphide screen. A screen
of Sidot's hexagonal blend (phosphorescent crystalline zinc
sulphide) lights up brightly under the action of the [Greek: alpha] rays of
radium and polonium. If the surface of the screen is examined
with a magnifying glass, the light from the screen is found not to
be uniformly distributed but to consist of a number of scintillating
points of light. No two flashes succeed one another at the same
point, but they are scattered over the surface, coming and going
rapidly without any movement of translation. This remarkable
action of the radium and polonium rays on a zinc sulphide screen
was discovered by Sir William Crookes[1], and independently by
Elster and Geitel[2], who observed it with the rays given out from
a wire which has been charged negatively either in the open air
or in a vessel containing the emanation of thorium.
In order to show the scintillations of radium on the screen, Sir William Crookes has devised a simple apparatus which he has called the "Spinthariscope." A small piece of metal, which has been dipped in a radium solution, is placed several millimetres away from a small zinc sulphide screen. This screen is fixed at one end of a short brass tube and is looked at through a lens fixed at the other end of the tube. Viewed in this way, the surface of the screen is seen as a dark background, dotted with brilliant points of light which come and go with great rapidity. The number of points of light per unit area to be seen at one time falls off rapidly as the distance from the radium increases, and, at several centi-*
