At the time of the discovery of radio-activity, about seventy-five substances were called elements; in other words, about seventy-five different substances were known to chemists, none of which had been separated into unlike parts, none of which had been made by the coalescence of unlike substances. Compounds of only two of these substances, uranium and thorium, are radio-active. Radio-activity is a very remarkable phenomenon. So far as we know at present, radio-activity is not a property of the substances which form almost the whole of the rocks, the waters, and the atmosphere of the earth; it is not a property of the materials which constitute living organisms. It is a property of some thirty substances--of course, the number may be increased--a few of which are found widely distributed in rocks and waters, but none of which is found anywhere except in extraordinarily minute quantity. Radium is the most abundant of these substances; but only a very few grains of radium chloride can be obtained from a couple of tons of pitchblende. In Chapter X. of _The Story of the Chemical Elements_ I have given a short account of the outstanding phenomena of radio-activity; for the present purpose it will suffice to state a few facts of fundamental importance. Radio-active substances are stores of energy, some of which is constantly escaping from them; they are constantly changing without external compulsion, and are constantly radiating energy: all explosives are storehouses of energy which, or part of which, can be obtained from them; but the liberation of their energy must be started by some kind of external shock. When an explosive substance has exploded, its existence as an explosive is finished; the products of the explosion are substances from which energy cannot be obtained: when a radio-active substance has exploded, it explodes again, and again, and again; a time comes, sooner or later, when it has changed into substances that are useless as sources of energy. The disintegration of an explosive, started by an external force, is generally completed in a fraction of a second; change of condition changes the rate of explosion: the "half-life period" of each radio-active substance is a constant characteristic of it; if a gram of radium were kept for about 1800 years, half of it would have changed into radio-inactive substances. Conditions may be arranged so that an explosive remains unchanged--wet gun-cotton is not exploded by a shock which would start the explosion of dry gun-cotton--in other words, the explosion of an explosive can be regulated: the explosive changes of a radio-active substance, which are accompanied by the radiation of energy, cannot be regulated; they proceed spontaneously in a regular and definable manner which is not influenced by any external conditions--such as great change of temperature, presence or absence of other substances--so far as these conditions have been made