Page:Radio-activity.djvu/323

 with sand or emery paper. If a negatively charged wire is placed in the presence of a large quantity of radium emanation, it becomes intensely active. If the wire, after removal, is drawn across a screen of zinc sulphide, or willemite, a portion of the active matter is rubbed off, and a luminous trail is left behind on the screen. The amount of active matter deposited is extremely small, for no difference of weight has been detected in a platinum wire when made extremely active. On examining the wire under a microscope, no trace of foreign matter is observed. It follows from these results that the matter which causes excited activity is many thousand times more active, weight for weight, than radium itself.

It is convenient to have a definite name for this radio-active matter, for the term "excited activity" only refers to the radiation from the active matter and not to the matter itself. The term "active deposit" will be generally applied to this matter. The active deposit from the three substances thorium, radium, and actinium is, in each case, derived from its respective emanation, and possesses the same general property of concentration on the negative electrode in an electric field and of acting as a non-volatile type of matter which is deposited from the gas on to the surface of bodies. These active deposits, while all soluble in strong acids, are chemically distinct from each other.

The term "active deposit" can, however, only be used when the matter is spoken of as a whole; for it will be shown later that the matter, under ordinary conditions, is complex and contains several constituents which have distinctive physical and chemical properties and also a distinctive rate of change. According to the theory advanced in section 136, we may suppose that the emanation of thorium, radium, and actinium is unstable and breaks up with the expulsion of an α particle. The residue of the atom of the emanation diffuses to the sides of the vessel or is removed to the negative electrode in an electric field. This active deposit is in turn unstable and breaks up in several successive stages.

The "excited activity" proper is the radiation set up by the active deposit in consequence of the changes occurring in it. On this view, the emanation is the parent of the active deposit in the same way that Th X is the parent of the emanation. The