The object of this brief treatise is to give a simple account of the development of our knowledge of radio-activity and its bearing on chemical and physical science. Mathematical processes will be omitted, as it is sufficient to give the assured results from calculations which are likely to be beyond the training of the reader. Experimental evidence will be given in detail wherever it is fundamental and necessary to a confident grasp of some of the marvelous deductions in this new branch of science. Theories cannot be avoided, but the facts remain while theories grow old and are discarded for others more in accord with the facts.

As so often happens in the history of science, the opening up of this new field with its fascinating disclosures was due to an investigation undertaken for another purpose but painstakingly carried out with a mind open to the truth wherever it might lead. In 1895, RÖntgen modestly announced his discovery of the X rays. This attracted immediate and intense interest. Among those who undertook to follow up these phenomena was Becquerel, who, because of the apparent connection with phosphorescence, tried the action of a number of phosphorescent substances upon the photographic plate, the most striking characteristic of the X rays being their effect upon such sensitive plates. In these experiments he obtained no results until he tried salts of uranium, recalling previous observations of his as to their phosphorescence. Distinct action was noted. Furthermore, he proved that this had no connection with the phenomenon of phosphorescence, as both uranic and uranous salts were active and the latter show no phosphorescence. Becquerel announced his discoveries in 1896 and this was the beginning of the new science of radio-activity.

The rays given off by uranium and its salts were found to differ from the X rays. They showed no appreciable variation in intensity, no previous exposure of the substance to light was necessary, and neither changes of temperature nor any other physical or chemical agency affected them.

At first uranium and its compounds were the only known source of these new radiations, but many other substances were examined and two years later thorium and its compounds were added to the list. In general the discharging action seemed about the same. Other elements and ordinary substances show a minute activity. Only potassium and rubidium have a greater activity than this, and theirs is only about one-thousandth that of uranium.

In the examination of uranium and thorium compounds it was found that the activity was determined by the uranium and thorium present; it was proportto the amount ofional these elements present and independent of the nature of the other elements composing the compound. The conclusion was, therefore, that the activity was an inherent property of the atoms of uranium and thorium, that is, an atomic property. This was a long step forward and introduced into science the conception of a new property of matter, or at least of certain forms of matter.

In examining a large number of minerals containing uranium and thorium, Mme. Curie made the important observation that many of these were more active than the elements themselves. In measuring the activity she made use of the electrical method which will be described later. In the following table giving her results for uranium minerals the numbers under i give the maximum current in amperes. They serve simply for comparison.

The last three are pure uranium and compounds of that element given for comparison with the first five, which are naturally occurring minerals. The last compound has the same composition as chalcolite and is simply the artificially prepared mineral. It has the activity which would be calculated from the proportion of uranium present, the copper and phosphoric acid contributing no activity.

Since the activity is not dependent upon the composition but upon the amount of uranium present, the activity in all of the minerals should be less than that of uranium. On the contrary, it is several times greater. Natural and artificial chalcolite also show a marked difference in favor of the former. The supposition was a natural one, therefore, that these minerals contained small quantities of an element, or elements, undetected by ordinary analysis and having a much greater activity than uranium. Similar results were obtained in the examination of thorium minerals and thorium salts.

Following up this supposition, M. and Mme. Curie set themselves the task of separating this unknown substance. Starting with pitchblende, a systematic chemical examination was made. This is an exceedingly complex mineral, containing many elements. The processes were laborious and demanded much time and minute care. They need not be described here. It is sufficient to say that along with bismuth a very active substance was separated, to which Mme. Curie gave the name of polonium for Poland, her native land. Its complete isolation is very difficult and sufficient quantities of the pure substance have not been obtained to determine its atomic weight and other properties, but some of the lines of its spectrum have been determined. Chemically it is very closely analogous to bismuth.

In a similar manner a barium precipitate was obtained from pitchblende which contained a highly active substance. The pure chloride of this body and barium can be prepared together and then separated by fractional crystallization. To the new body thus found the name of radium was given. It is similar in chemical properties to barium. Its atomic weight has been determined by several careful investigators and is accepted as 226. Its spectrum has been mapped and its general properties are known. It is a silvery white, oxidizable metal. In one ton of pitchblende about 0.2 gram of radium is present; this is about 5000 times greater than the amount of polonium present. The activity of the products was depended upon as the guide in these separations. The radium found is relatively enormously more active than the pitchblende or uranium.

In the above separations use was made of relationships to bismuth and barium. Similarly, by taking advantage of chemical relationship to the iron group of elements, another body was partially separated by Debierne, to which he gave the name actinium. Boltwood discovered in uranium minerals the presence of a body which he named ionium, and which is so similar to thorium that it cannot be separated from it. It, however, far exceeds thorium in activity.

The lead which is present in uranium and thorium minerals—apparently in fairly definite ratio to the amount of uranium and thorium—is found, on separation and purification, to possess radio-active properties. This activity is due to the presence of a very small proportion of an active constituent called radio-lead, which has chemical properties identical with those of ordinary lead. The bulk of the lead obtained from radio-active minerals differs in atomic weight from ordinary lead and appears also to be different according to whether its source is a thorium or a uranium mineral.

A large number of other radio-active substances have been separated and some of their properties determined, but these were found by different means and will be noted in their proper place. They number in all more than thirty. The sources or parents of these are the original uranium or thorium, and the products form regular series with distinctive properties for each member.