The Emanation from Radium.
The examination of radium revealed a behavior exactly similar to that of uranium, for it too was found to be continually producing a radio-active substance which, when separated from the radium, slowly lost its activity, while the radium from which it was separated slowly regained its original radiating power. In the case of radium this new substance, unlike the uranium X and the thorium X, could be distinguished by other physical properties besides its activity. Thus Rutherford found it to be of the nature of a gas. It could be separated from radium by heating the latter, or by dissolving it in water. The radium which had been so treated lost for the time being all but one fourth of its original radiating power, the other three fourths being found in the gas, or emanation, as Rutherford called it. This gas could be carried by means of air currents through long tubes to considerable distances from the radium itself, its path through the tubes being easily traced by the fluorescence which it imparted to the glass walls of the tubes. It could be set away in bottles and the change in its activity watched from day to day. In this way it was found to lose about half its activity in a period of four days, while in the same period the radium from which it had been separated regained one half of its lost radiating power. By passing this gas or emanation through a tube immersed in liquid air, Rutherford found that it condensed at about -150° C. Ramsay has recently found that it appears to have a characteristic spectrum, as have all the elements. This gas, therefore, seems to be a substance of very definite physical qualities which is produced by the disintegration of the atom of radium in just the same way as the uranium X and thorium X are produced by the disintegration of the atoms of uranium and thorium. But this gas, like the uranium X and the thorium X, has but a transitory existence, for the fact that it gradually loses its activity shows that it passes on into something else.
Induced Radio-activity.
Nor did physicists have long to look in order to discover this substance into which the emanation from radium is transformed. The Curies found as early as 1899 that when this gas comes into contact with a solid object, the object becomes coated with a film of radioactive matter which can be dissolved with hydrochloric or sulphuric acid, and which is left in the dish when the acid is evaporated. Or which may be rubbed off with leather and found, by means of the property of activity which it possesses, in the ash of the leather after the leather has been burned. This radio-active matter is so infinitesimal in amount that in no case is it detected in any other way than by its radio-activity. It might, at first, look as though it were nothing but the active gas itself condensed on the surface of the solid object, but since the rate at which it loses its activity is altogether different
