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.
These effects and their connection with the emanation are discussed more fully in chapter VIII.
