Page:Radio-activity.djvu/419

 '''230. Active deposit of radium of slow transformation.''' It has been pointed out (section 183) that a body, exposed in the presence of the radium emanation, does not lose all its activity for a long time after removal; a small residual activity is always observed. The magnitude of this residual activity is dependent not only upon the amount of emanation employed, but also upon the time of exposure of the body in the presence of the emanation. For an exposure of several hours in the presence of the emanation, the residual activity is less than one-millionth of the activity immediately after removal.

An account will now be given of some investigations made by the writer on the nature of this residual activity and the chemical properties of the active matter itself. It is first of all necessary to show that the residual activity arises in consequence of a deposit of radio-active matter, and is not due to some action of the intense radiations to which the body made active has been subjected.

The inside of a long glass tube was covered with equal areas of thin metal, including aluminium, iron, copper, silver, lead, and platinum. A large amount of radium emanation was introduced into the tube, and the tube closed. After seven days the metal plates were removed, and, after allowing two days to elapse for the ordinary excited activity to disappear, the residual activity of the plates was tested by an electrometer. The activity of the plates was found to be unequal, being greatest for copper and silver, and least for aluminium. The activity of copper was twice as great as that of aluminium. After standing for another week the activity of the plates was again tested. The activity of each had diminished in the interval to some extent, but the initial differences observed had to a large extent disappeared. After reaching a minimum value the activity of each plate slowly but steadily increased at the same rate. After a month's interval the activity of each of the plates was nearly the same, and more than three times the minimum value. The initial irregularities in the decay curves of the different metals are, in all probability, due to slight but different degrees of absorption of the radium emanation by the metal plates, the absorption being greatest for copper and silver and least for