=146.= Debierne[240] found that actinium gives out an emanation similar to the emanation of thorium and radium. The loss of activity of the emanation is even more rapid than for the thorium emanation, for its activity falls to half value in 3·9 seconds. In consequence of the rapid decay of activity, the emanation is able to diffuse through the air only a short distance from the active matter before it loses the greater proportion of its activity. Giesel early observed that the radio-active substance separated by him, which we have seen (section 18) is identical in radio-active properties with actinium, gave off a large amount of emanation. It was in consequence of this property, that he gave it the name of the “emanating substance” and later “emanium.” The impure preparations of this substance emit the emanation very freely and in this respect differ from most of the thorium compounds. The emanation from actinium like those from thorium and radium possesses the property of exciting activity on inactive bodies, but it has not yet been studied so completely as the better known emanations of thorium and radium. Experiments with large amounts of Radium Emanation. =147.= With very active specimens of radium a large amount of emanation can be obtained, and the electrical, photographic, and fluorescent effects are correspondingly intense. On account of the small activity of thorium and the rapid decay of its emanation the effects due to it are weak, and can be studied only for a few minutes after its production. The emanation from radium, on the other hand, in consequence of the slow decay of its activity, may be stored mixed with air in an ordinary gas-holder, and its photographic and electrical actions may be examined several days or even weeks after, quite apart from those of the radium from which it was obtained. It is, in general, difficult to study the radiation due to the emanation alone, on account of the fact that the emanation is continually producing a secondary type of activity on the surface of the vessel in which the emanation is enclosed. This excited activity reaches a maximum value several hours after the introduction of the emanation, and, as long as it is kept in the vessel, this excited activity on the walls decays at the same rate as the emanation itself, _i.e._ it falls to half its initial value in about 4 days. If, however, the emanation is blown out, the excited activity remains behind on the surface, but rapidly loses its activity in the course of a few hours. After several hours the intensity of the residual radiation is very small.