NEWSPAPER-READERS are continually dropping across references to the radio-activity of different substances. But there is, as a rule, a general vagueness as to what is exactly implied by the term. Unless one has access to the very latest works on science, or is a practical experimenter himself, this vagueness is only natural.

      Practically, the far-reaching discoveries that have been made in this subject have all been accomplished within the last ten years. Twenty years ago the names were unknown — there was no suspicion of the great field that has been but recently revealed.

Seas We Live In.

      These rays are, of course, not new. They existed ever since the world began. But their existence was never suspected, for they are all invisible. For thousands of centuries we have lived in an intricate sea of invisible and unfelt waves, simply because our organs of sense are incapable of perceiving them. And when our theories of things were built, all these were never taken into account — till five years ago.

      The first echo that reached our ears was when Röntgen accidentally stumbled across the now famous X-rays. That discovery was the bomb-shell whose explosion startled the scientific world into activity. The credit must not be grudged because Röntgen was first put upon the track by an accident. He was experimenting with electrical discharges in-vacuo, and, luckily, had a box containing a spare photographic plate lying about. Later on, when he picked it up, he noticed that it was smudged. Instead of throwing it on one side, he commenced a series of experiments which ended nine years ago in the announcement to the almost incredulous world of the existence of previously unknown rays, which he called the X-rays.

      These Röntgen rays are an invisible light — vibrations set up in the all-pervading ether. They speed in undulating waves outward, with a velocity that baffles comprehension. Quivering millions of millions of times in each separate second, they leave the mind stupefied with their inconceivable rapidity. They are of deep interest to science, because, being the swiftest known motion in Nature, they possess a remarkable power of penetration. Unlike other forms of light, they are not stopped by sheets of paper or cloth, but pass straight through.

Light from the Human Body.

      One of the few substances that can stop these rays is a human bone — a happy property, which has proved invaluable to surgeons. Radiographs of our limbs, showing where fractures occur or bullets have lodged, are now familiar.

      The next advance into the unknown was made by Becquerel, also with a photographic plate. Six years ago he was investigating uranium salts in Paris, and discovered that they also were capable of affecting a sensitive film through paper. This opened out quite a new aspect of rays. So far, the X-rays had been artificially produced by sending electrical, sparks through Crookes' tubes, but now it was found that some substances naturally generated them, though in lesser degree. These Becquerel rays are closely allied to the artificial X-Rays, and close observation, aided by the most delicate instruments, revealed the fact that most substances emit these rays, but in varying quantities. The metals — especially the heavier ones — were appreciably radiating small quantities of this invisible light.

      Different sorts of rays were also discovered in other directions. It was found that whenever a bar of metal was put into a state of stress, such as that induced by bending it, from the bent part another type of rays poured out. Lastly, the human body was discovered by Charpentier to also emit a special vibratory ray. These two particular rays are known as the Blondlot and N-rays.

Madame Curie's Miracle Shop.

      Those from the human body are not as yet accepted unconditionally; they are extremely difficult to detect. But they find strong corroboration in the forgotten researches of Reichenbach a century ago. He discovered in hypnotic tests that the subject saw a light flowing from the hands of the hypnotist, and surrounding him like a halo.

      About three years ago Madame Curie investigated the cause of radio-activity of uranium, and discovered that it was mostly due to the presence of an hitherto unknown element. She succeeded in separating it, and presented to the world the celebrated radium. Overcoming enormous chemical difficulties, she and her husband isolated a small fragment from tons of pitchblende ore. Radium is the most powerfully radio-active element known, and, apart from the curious fact that it evolves heat waves, which keeps it always slightly warmer than the air, it emits no less than three different kinds of rays. These are named after the first three letters of the Greek alphabet.

      If a sheet of tinfoil or mica is interposed in their path, the Alpha rays arc stopped; an inch sheet of lead farther on will check the Beta rays — they cannot pierce through; but, so far, nothing has been discovered that can arrest the frightful velocity of the Gamma rays — they penetrate everything. As these rays rush outward a heavy gas is left behind.

One Pound Equals 250 Tons.

      When subjected to examination, the Alpha rays proved to be charged with positive electricity, whilst the Beta rays are negative in their action and almost identical with the older X-rays. When these rays strike on a fluorescent screen they cause the brilliant, bursting flashes of light that are readily seen in the little instrument called Crookes' spinthariscope. It is not yet settled whether the flash is caused by the tiny electron shattering itself on the zinc-sulphide crystals, or whether in the presence of these rays the crystal itself bursts into fragments. The action suggests a furious bombardment.

      The Gamma rays have been so recently identified that our knowledge about them is practically nil. An idea, however, of the force that lies in radium may be formed from Sir William Ramsay's statement that one pound of the gas, in heat alone, is equal to producing the energy of 250 tons of dynamite.