The rapid and continuous emission of this short-lived emanation is the most striking radio-active property possessed by actinium. In still air, the radio-active effects of this emanation are confined to a distance of a few centimetres from the active material, as it is only able to diffuse a short distance through the air before losing its radiating power. With very active preparations of actinium, the material appears to be surrounded by a luminous haze produced by the emanation. The radiations produce strong luminosity in some substances, for example, zinc sulphide, willemite and platinocyanide of barium. The luminosity is especially marked on screens of zinc sulphide. Much of this effect is due to the emanation, for, on gently blowing a current of air over the substance, the luminosity is displaced at once in the direction of the current. With a zinc sulphide screen, actinium shows the phenomena of “scintillations” to an even more marked degree than radium itself. The preparations of emanium are in some cases luminous, and a spectroscopic examination of this light has shown a number of bright lines[31]. The distinctive character of the emanation of actinium, as well as of the other radio-active products to which it gives rise, coupled with the permanence of its activity, renders it very probable that actinium will prove to be a new radio-active element of very great activity. Although very active preparations of actinium have been obtained, it has not yet been found possible to free it from impurities. Consequently, no definite observations have been made on its chemical properties, and no new spectrum lines have been observed. A more complete discussion of the radio-active and other properties of actinium is given in later chapters. =19. Polonium.= Polonium was the first of the active substances obtained from pitchblende. It has been investigated in detail by its discoverer Mme Curie[32]. The pitchblende was dissolved in acid and sulphuretted hydrogen added. The precipitated sulphides contained an active substance, which, after separation of impurities, was found associated with bismuth. This active substance, which has been named polonium, is so closely allied in chemical properties to bismuth that it has so far been found impossible to effect a complete separation. Partial separation of polonium can be made by successive fractionations based on one of the following modes of procedure: (1) Sublimation in a vacuum. The active sulphide is more volatile than that of bismuth. It is deposited as a black substance at those parts of the tube, where the temperature is between 250 and 300° C. In this way polonium of activity 700 times that of uranium was obtained. (2) Precipitation of nitric acid solutions by water. The precipitated sub-nitrate is much more active than the part that remains in solution.