RECENT DISCOVERIES IN RADIATION. 491 



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. Eadium 

 itself, in the dark, glows with a light which resembles that of a glow- 

 worm, 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 soiree of the Eoyal 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 micro- 

 scope 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 fiash 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 hy 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 heta 

 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 tem- 

 perature 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 tem- 

 perature 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 sub- 

 stances, 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 



