The a-radio-ac-tivity of thorium has been found by Hahn to be due to the presence in it of a substance giving reactions similar to those of thorium ; this radiothorium can be separated into thorium X, a substance of which the half-period of decay is 4 days ; evolving a-rays, it changes into thorium emanation, a gas of which the half-life period is 54 seconds; then follow in succession thorium A, B, and C, the last emitting a-, /?-, and y-rays, and giving an unknown product, conceivably lead. It is still doubtful whether thorium emanation yields helium.

Actinium,-As already stated, this element resembles yttrium in its chemical reactions. It has never been obtained in sufficient quantity to make a chemical examination possible. Emitting a-rays, it changes (half-period about 20 days) into radio actinium ; again emitting a-rays, the product is actinium X, of which the half-life period is 10.2 days; this in its turn gives actinium emanation, which possesses the short half-life period of 3.9 seconds. These changes are so rapid that if some impure oxides containing actinium be held under a piece of pasteboard, covered with a layer of hexagonal zincblende (ZnS) or of barium platinicyanide (BaPt(CN)_G), a brilliant phosphorescence is observable, due to the bombardment of the screen with a-rays. The emanation, evolving heat, rises, although its density is almost certainly much greater than that of air. The subsequent changes are: actinium emanation into actinium A (36 minutes) ; and A into B (3 minutes). The final product is unknown. It has been found by Debierne that like radium emanation, actinium emanation in the gaseous state yields helium as one of its products.

Although the actual amount of energy evolved during the spontaneous changes mentioned has been measured only in the case of radium and its emanation, it is in the highest degree probable that a similar enormous evolution is also a concomitant of other disintegrations. Now, bodies whose changes occur with evolution of energy may be likened to endothermic elements or compounds; the reader may be reminded of monoclinic sulphur, which evolves heat and changes into the rhombic allotropic modification. It may be supposed that radium, to take a concrete example, in changing to the emanation, evolves heat, while its product, the emanation, is formed. It is also unstable, however, and in its turn yields radium A; that substance, radium B, etc. We may again borrow a partial analogy. Hypochlorous acid, HClO.Aq, changes to chloric acid, HC1O₃.Aq, with evolution of energy; that again to C1O₂ + HC1O₄, with a further loss ; and finally the products are Cl₂, HC1, and O₂. The intermediate products HC1O₃, C1O₂, and HC1O₄ are " metastable," i.e. capable of existence, but easily put " off the balance," and yielding stable bodies at the last.

At the same time, it is possible to produce a metastable compound, i.e. a compound produced by absorption of energy, only if its formation is accompanied by the simultaneous formation of an exothermic substance. Thus when Cl₂ + ₂NaOH.Aq « NaCl.Aq + H₂O + NaOCl.Aq, the exothermic compounds, NaCl and H₉O, give off more energy than is absorbed during the formation of the endothermic NaOCl.

Reasoning from analogy, it would seem not unlikely that the change of radium emanation into RaA is an exothermic reaction; and that the formation from a portion of the emanation of helium, neon, or argon, under different circumstances is endothermic; similarly, the change of copper into lithium. But further research is obviously required before this provisional working hypothesis can be accepted.

In conclusion, it may be pointed out that the progress of chemical discovery has been largely due to the increase of knowledge how to concentrate and direct energy. Thus, to take only one example, Davy's discovery of the alkali-metals was due to his effecting a concentration of energy, at that time enormous, at the end of two platinum wires. Compared even with that, the radium emanation may be regarded as a prodigious source of energy; and the chemical changes which it is capable of producing will doubtless be still more radical.