It has been found experimentally that the property possessed by thorium of giving off an emanation belongs not directly to the thorium but to its product, thorium X. The amount of the emanation is always proportional to the amount of thorium X present, and thorium freed &ova thorium X does not emit any.emana tion. This shows that thorium X is the parent of the emanation. According to the disintegra tion theory, advanced by Rutherford and Soddy, the atom of the emanation arises as a result of the expulsion of a particle from the atom of thorium X. In a similar way it has been shown that the emanation is transformed into the active deposit which has been found to consist of a number of distinct substances, four in all, which are successive products 'formed from the thorium emanation. These con stituents can be separated from one another by electrolysis, or by utilizing the differences in their volatility when subjected to heat.
The process of analysis which has been ap plied to thorium has been applied with equal success to uranium, radium and actinium. Sir William Crookes found that uranium has a product called uranium X, which is half trans formed in 22 days and gives out only B and Y rays. The product actinium X was discovered by The production of a series of disintegration products amounting to seven in all has been observed in the case of actinium. It is impossible in this short article to discuss in detail the experiments by which the presence and properties of these series of products, re sulting from the transformation of the radio elements, have been determined.
It will be seen that seven distinct transforma tion products have been found in radium. The active deposit of rapid change arising from the emanation consists of three substances, called radium A, B and C, which are half transformed in 3, 27 and 19 minutes, respectively. After these products have undergone transformation, a residue of very slight activity remains. The properties of this residue were examined by Rutherford, who showed that it breaks up through three successive stages, 'called radium D, E and F. The substance Indiana D gives out soft beta rays, and is transformed very slowly —about half value in 16 years. The examina tion of this residue brought to light the very important fact that polonium is a product of the disintegration of radium. The product radium F is identical with polonium, for their chemical and physical properties are the same, and both lose their activity at the same rate. It has also been shown that radium D is the primary substance, which causes the activity ob served in the radio-lead of Hofmann. The ac tive substances present in polonium, and radio lead are thus proved to be members of the radium family.
The products obtained from the radio-ele ments all possess some distinctive physical and chemical properties, which distinguish them not only from other products but also from the parent element. For example, radium—a solid substance with chemical properties closely allied to barium—is transformed into a gaseous sub stance, the radium emanation. The emanation in turn is transformed into a solid. Each of these products must be considered a distinct chemical substance. They differ from the ordi nary chemical elements in the fact that they are unstable and undergo transformation into other or new kinds of matter. On account of their very limited life, these products exist in very minute quantity, and it is only in the case of a comparatively slowly changing product like the radium emanation that a sufficient amount can be obtained to measure. There is no doubt,
however, that both radium D and F will ulti mately be obtained in sufficient quantity to ob tain their spectrum and chemical constants.
In the radio-elements, we are witnessing the transformation of matter through a series of successive stages. The products represent the halting places where the matter is able to exist a short time before it breaks down into new forms. This process of transfortnation is spon taneous and, as far as observation has at present gone, cannot be hastened or retarded by any physical or chemical agencies.
Since the radio-elements are steadily being transformed, the original matter must slowly 'disappear and be changed into other ldnds. It can be calculated with some confidence that radium will be half transformed in about years. A feebly active substance like uranium will take several thousand million years to be half transformed. The radio-elements ap parently only differ from•their products in their length of life; that is, in the stability of the atoms composing them.
A mass of radium left by itself would, in the course of 20,000 years, be almost completely transformed. Since the age of the minerals in which radium is found is known to be much older than this, • it is, necessary to suppose that radium is constantly produced from some other substance. The ultimate parent of radium is the heavy element uranium, with which it is always found associated. Such a view was strongly confirmed by investigations of Bolt wood, Strutt and McCoy. Boltwood found that the amount of radium in different radioactive minerals, obtained from different parts of the world, is always proportional to their content of uranium. Such a relation indicates that radium is derived from uranium. If this is the case, a quantity of uranium, initially freed from radium, should in the course of time produce some radium. Such a growth of radium from uranium has actually been recorded in some ex periments by Soddy and Whetham. The rate of production observed is, however, much slower than is to be expected if uranium passes directly into radium. It has been, shown by Boltwood that an intermediate substance, iom.um, is first produced and that this in turn changes into radium. The evidence is thus very strong that one element uranium produces another element ionium, which then produces radium. Each of these radioactive products is quite distinct in physical and chemical properties from its parent The radioactive minerals must be considered as nature's laboratory in which all the products of transformation of the radioactive substances will be found. Rutherford and Soddy early suggested that the rare gas helium, which is always found . associated with the radioactive minerals was a transformation product of the radio-elements. This prediction was verified by some very striking experiments made by Ram say and Soddy. They removed the emanation from a quantity of radium and collected it in a tube. A discharge was passed through this tube and the light examined by a spectroscope. Alter standing several hours the spectrum of helium was observed, showing that this gas had been produced during the transformation of the emanation. Here again we have an example of the production of one element helium from another.