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I NTO a hollow sphere of unit radius, with a curvature or bend, therefore,-I, put two solid spheres of radius t. or bend + 2. The two solid spheres then kiss each other at the exact centre of since it has the unique property that the bend of every sphere of the infinitely infinite number of spheres that theoretically can be packed into it, so that each is located by its neighbours, is an exact :Fig. 1.

practically all of the emanation comes off very suddenly at a temperature not much more than one degree above that at which only 2 per cent. has volatilized. The general indication of all the experiments, considered together, is to show that the condensed emanation possesses a true vapour-pressure, and that the emanation commences to volatilize slowly two or three degrees below the temperature of rapid volatilization even when the process occurs in a stationary atmosphere. The emanations therefore possess ~he usual properties possessed by ordinary gaseous matter, in so far as the phenomena of volatilization and condensation are concerned. It was shown in a recent paper that they also possess the property possessed by gases of being occluded by solids under certain conditions. These new properties, taken in conjunction with the earlier discovered diffusion phenomena, characteristic of the radioactive emanations, leave no doubt that the latter must consist of mat~;er in the gaseous s~ate.

ECXXI V.-The Radioactivity of Tkorium Compounds-11-The Cause and Nature of Radioactivity. By E. RUTHERFORD and FREDERICK SODDP. AT the close of the first paper on this subject (tbis vol., p. 321), it was shown that a constituent responsible for part of the radiohtivity of thorium could be separated from its compounds by chemical means. TWO methods were given. In one, a thorium solution was precipitated by ammonia, and the thorium hydroxide precipitate showed only about one-third of the normal activity, whilst the filtrate, on evaporation and removal of the ammonium salts by ignition, left a very active residue i n some cases more than a thousand times as active as an equal weight of thoria. In the other method, thorium oxide was washed repeatedly with large quantities of water. The washings, on evaporation, deposited very active residues, whilst the radioactivity of the thoria was appreciably diminished by the process. In both these methods, the active residues are extremely small, and the view was put forward that even the most active specimens consisted largely of accidental impurities, ThX, the active constituent of thorium, being only present in minute amount. By the kindness of Dr. KnGfler, of Berlin, who in the friendliest manner placed at our disposal a large specimen of his purest thorium nitrate, we were a t once able to confirm this opinion, This specimen, which had been purified by many processes, did not contain any of the impurity precipitable by sodium phosphate after the removal of the thorium with ammonia, which was present in the commercial nitrate previously used. The radioactivity and emanating power of Dr. Kniifler's specimen were, however, at least as great aa any other in our possession, and the residuerJ obtained from the filtrate after precipitating with ammonia were no less active.

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