sist, largely at least, of projected particles of matter expelled with enormous velocities from the active substance. But no amount of reasoning of the sort thus far given will be found half as convincing to the ordinary mind as the sight of a bit of radium at work. Radium itself, in the dark, glows with a light which resembles that of a glowworm, and when placed near certain substances like willemite (zinc silicate) or zinc sulphide, it causes them to light up with a glow which is more or less brilliant according to the amount of the radium at hand. Last spring Sir William Crookes first exhibited the following most beautiful and wonderful experiment at the soirée of the Royal Society in London. A small bit of radium is placed about a millimeter above a zinc sulphide screen, and the latter is then viewed through a microscope of from ten to twenty diameters magnification. The continuous soft glow of the screen, which is all that one sees with the naked eye, is resolved by the microscope into a thousand tiny flashes of light. It is as though one were viewing a swamp full of fire flies, or, better still, a sky full of shooting stars. The appearance is as though the screen were being fiercely bombarded by an incessant rain of projectiles, each impact being marked by a flash of light, just as sparks fly off from an iron when it is struck with a hammer. Becquerel has recently brought forward evidence to show that the spark is due to a cleavage produced in the zinc sulphide crystal by the impact of the alpha particles. This explains why the effect is not observable with all kinds of screens.
The Continuous Emission of Light and Heat by Radio-active Substances.
After learning that the radio-active substances uranium, thorium and radium are, for some reason or other, continuously projecting with enormous velocities two kinds of particles, the alpha and the beta particles, one is not surprised to find that these substances maintain a temperature above the temperature of the surrounding atmosphere. This has been proved experimentally only for radium, which was found last year by M. Curie and M. Laborde to remain permanently at a temperature between one and two degrees centigrade above that of its surroundings, and to give out for each gram of weight enough heat per hour to raise a hundred grams of water through one degree. Since radium radiates more than a million times more actively than either of the other substances, it is not likely that any one will ever be able to show experimentally that uranium and thorium also maintain a temperature above that of their surroundings. Nevertheless, the same causes which operate to hold up the temperature of radium, operate also to hold up the temperature of both the other radio-active substances, the only difference being one of degree. Hence it is probable that all radio-active substances are continuously emitting, in a greater or less degree, heat energy. This is not surprising in view of the
