Polonium can be concentrated by fractionally precipitating the subnitrate of polonium and bismuth by means of water. The part precipitated is the most active. One can also make a partial precipitation by means of hydrogen sulphide from a strongly acid solution in hydrochloric acid. These methods of fractionation are difficult because the precipitates are not easily redissolved. Marckwald concentrated the activity by plunging a rod of bismuth in a solution of bismuth and polonium. A layer of extremely active metal was deposited upon the rod.
Actinium.—The concentration of actinium is even more difficult than that of polonium. Solid salts containing actinium preserve their radioactivity unchanged for several years.
II. The Radiations from Radioactive Substances.
Complexity of the Radiation.—Radium is the substance whose radioactivity has been studied most completely. It is now known that it emits a number of rays of different nature, which can be placed in three groups. According to the notation adopted by Rutherford, I designate the three groups of rays by the letters α, β, and γ.
The action of the magnetic field serves to distinguish them. In an intense magnetic field the α-rays are slightly deviated from a straight path, and the deviation is of the same kind as that of the "canal" rays of Goldstein in a vacuum-tube. On the other hand, the β-rays are deviated like the cathode rays, and the γ-rays are not turned aside, but act like the Roentgen rays.
β-Rays.—The β-rays of radium, analogous to the cathode rays, form a heterogeneous group. They are distinguished from one another by their power of penetration and by the deviation caused by a magnetic field.
Certain β-rays are absorbed by aluminium foil a few hundreths of a millimeter in thickness, while others pass through several millimeters of lead.
Suppose we have a rectilinear pencil of Becquerel rays obtained by means of a particle of radium salt and a screen with a hole in it. If a uniform magnetic field is produced normal to the direction of the pencil the β-rays are incurved, and describe circular paths in a plane normal to the direction of the magnetic field. The rays of the described circumferences vary within wide limits. Becquerel has shown that the most penetrating rays are least deviated, and hence describe circumferences with the greatest radius of curvature. If the pencil of β-rays bent aside by the magnetic field is allowed to fall on the photographic plate, there is obtained an impression that is a true spectrum in which the different β-rays act separately.
Suppose the β-rays to be projectiles (electrons) negatively charged with electricity and shot from the radium with great velocity. Let
