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Updated: June 17, 2025
The first definite discovery of such a radio-active substance was made by M. Henri Becquerel, in 1896, while making some experiments upon the peculiar ore pitch-blende. Pitch-blende is a heavy, black, pitchy-looking mineral, found principally at present in some parts of Saxony and Bohemia on the Continent, in Cornwall in Great Britain, and in Colorado in America.
The discovery of the radio-active properties of uranium was followed about two years later by the discovery that thorium, and the minerals containing thorium, possess properties similar to those of uranium. This discovery was made independently and at about the same time by Schmidt and Madame Skaldowska Curie.
The alpha-rays from radio-active substances are swarms of tiny meteors flying at the incredible speed of twelve thousand miles a second, while the meteors of the midnight sky fly at the speed of only forty miles a second. Those alpha particles are helium atoms. They are much larger than beta particles, and have less penetrative power.
All these material designs are more or less radio-active namely, changing into other elements, but some, like radium, polonium, &c., are active to an extraordinary extent.
The radio-activity of salts of uranium was proved not to be increased or diminished when these salts had been shielded for five years from the action of light by keeping them in leaden boxes. Shortly after Becquerel's discovery, experiments proved that salts of the rare metal thorium are radio-active.
The assumption is that a radio-active substance, like uranium, "decays," or passes into many other substances, of which radium is one, finally producing lead in 1,000,000,000 years or more. From this theory, Prof. Russell concludes that the earth is 4,000,000,000 to 8,000,000,000 years old, and the sun is older still.
Since the earth and the atmosphere contain appreciable quantities of radio-active matter, this must play a very important part in determining the temperature of the globe so important a part, indeed, that all former estimates as to the probable length of time during which the earth and sun will continue to radiate heat are invalidated.
When a solution of a radium salt is placed in a closed vessel the radio-activity in part leaves the solution and distributes itself through the vessel, the sides of which become radio-active and luminous.
If one names the radio-active substances elements, one is placing in this class substances which are specially characterised by a property the direct opposite of that the possession of which by other substances was the reason for the formation of the class. To do this may be ingenious; it is certainly not scientific.
The radio-active bodies, however, are the only ones that have afforded an opportunity for studying this transformation. In these the rapidity of the change would be directly proportionate to their radioactivity. Radium, according to the recent estimate of the Curies, would be disintegrating over a million times more rapidly than uranium.
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