Twenty-eight elements are now classed in three divisions with the three parents, uranium, thorium, and actinium. Potassium and rubidium have been shown to be radio-active, but otherwise the alkaline metals do not enter the classes.
Describe radium and its special properties.
What It Is Like.—To the eye a tiny sample of radium—or, to speak more correctly, of one of the radium salts, for radium in a pure state (i.e. the metal) has not been obtained as yet—presents no very striking appearance. All one sees is a few tiny crystals, or perhaps a few specks of whitish-looking powder, glowing in the dark with a faint phosphorescent light similar to that sometimes emitted by a piece of decaying fish.
The Radiations are of three kinds, comparable with those of the vacuum tube: Alpha-rays are heavy particles, positively charged, similar to the canal rays; Beta-electrons, negative like cathode rays; Gamma-rays resemble Röntgen rays. They penetrate matter to different degrees, behave differently under the action of a magnetic field, but under ordinary circumstances travel in straight lines.
But rays from different elements vary in penetration, and also with the absorbing substance, varying roughly with the density.
The Alpha-rays have a velocity of from 1.56 × 109 centimeters per second (radium) to 2.25 × 109 centimeters per second (thorium); they are particles of helium carrying a double charge of electricity. Beta-rays have a greater range of velocity and approach that of light. Both [895] Alpha- and Beta-rays are absorbed by a thickness of one centimeter of lead, but Gamma-rays pass through an inch of lead; they carry no charge of electricity, yet ionize the air and discharge the electrometer.
All the rays on impinging on solid particles give rise to secondary rays, sometimes called Delta-rays, electrons moving with comparatively low velocity. The Alpha-rays possess ninety-five per cent of the energy evolved and produce brilliant fluorescence in zinc sulphide, diamond, etc., the other rays producing this best in willemite and the platino-cyanides; all become absorbed and transmuted into heat.
Radium every hour generates sufficient heat to raise its own weight of water from freezing to boiling point.
The Spinthariscope.—This is a simple piece of apparatus invented by Sir William Crookes, by means of which some of the effects of the Alpha-ray particles can be observed in a very striking manner. It consists of a little screen covered with powdered zinc sulphide. A small fragment of radium is placed directly in front of the middle of the screen and in close proximity to it. On observing this screen in the dark through a suitable lens, scintillating little points of light are seen to be continually flashing into view and dying away. Each tiny spark is thought to be produced by the impact of a single Alpha-ray particle. That these particles or emanations must be matter in a state of extreme attenuation is proved by an experiment of Professor Curie’s in which a box constructed of platinum was pierced with two holes so minute as to be capable of retaining a vacuum, and yet these radium emanations passed through quite freely.
What are the medical uses of radium?