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.