The evidence thus seems to be conclusive that Th X is the parent of the emanation and that the emanation is the parent of the deposited matter.
194. Chemical and Physical properties of the active products. Each of these radio-active products is marked by some distinctive chemical and physical properties which differentiate it from the preceding and succeeding products. For example, Th X behaves as a solid. It is soluble in ammonia, while thorium is not. The thorium emanation behaves as a chemically inert gas and condenses at a temperature of -120° C. The active deposit from the emanation behaves as a solid and is readily soluble in sulphuric and hydrochloric acids and is only slightly soluble in ammonia.
The striking dissimilarity which exists in many cases between the chemical and the physical properties of the parent matter and the product to which it gives rise is very well illustrated by the case of radium and the radium emanation. Radium is an element so closely allied in chemical properties to barium that, apart from a slight difference in the solubility of the chlorides and bromides, it is difficult to distinguish chemically between them. It has a definite spectrum of bright lines similar in many respects to the spectra of the alkaline earths. Like barium, it is non-volatile at ordinary temperature. On the other hand, the emanation which is continually produced from radium is a radio-active and chemically inert gas, which is condensed at a temperature of -150° C. Both in its spectrum and in the absence of definite chemical properties, it resembles the argon-helium group of inert gases, but differs from these gases in certain marked features.
The emanation must be considered to be an unstable gas which breaks down into a non-volatile type of matter, the disintegration being accompanied by the expulsion of heavy atoms of matter (α particles) projected with great velocity. This rate of breaking up is not affected by temperature over the considerable range which has been examined. After a month’s interval, the volume of the emanation has shrunk to a small portion of its initial value. But the most striking property of the emanation, which, as we shall see later (chapter XII), is a direct consequence of its radio-activity, is the enormous amount of energy emitted from it. The emanation in breaking up through its successive stages emits about 3 million times as much energy as is given out by the explosion of an equal volume of hydrogen and oxygen, mixed in the proper proportions to form water; and yet, in this latter chemical reaction more heat is emitted than in any other known chemical change.
We have seen that the two emanations and the products Ur X, Th X lose their activity with the time according to a simple exponential law, and at a rate that is independent—as far as observation has gone—of the chemical and physical agents at our disposal. The time taken for each of these products to fall to half its value is thus a definite physical constant which serves to distinguish it from all other products.
On the other hand, the variation of the excited activity produced by these emanations does not even approximately obey such a law. The rate of decay depends not only on the time of exposure to the respective emanations, but also, in the case of radium, on the type of radiation which is used as a means of comparative measurement. It will be shown, in succeeding chapters, that the complexity of the decay is due to the fact that the matter in the active deposits undergoes several successive transformations, and that the peculiarities of the curves of decay, obtained under different conditions, can be explained completely on the assumption that two changes occur in the active deposit from both thorium and actinium and six in the active deposit from radium.
195. Nomenclature. The nomenclature to be applied to the numerous radio-active products is a question of great importance and also one of considerable difficulty. Since there are at least seven distinct substances