Nature 70: 270 (1904)

In the course of some recent experiments on the excited radio-activity from the radium emanation, some evidence has been obtained which points to the conclusion that the emanation X of radium at one stage of the changes which it undergoes after being deposited on a solid body is slightly volatile even at ordinary temperatures. The effect which gives rise to this conclusion was first noticed in some observations on the rate of decay of the part of the excited activity deposited on a plate of copper immersed for a short time in dilute hydrochloric acid in which the activity from a platinum wire exposed for a time to the radium emanation had been dissolved. When the copper plate with its active deposit had been placed inside a testing vessel and removed after a few minutes, it was noticed that a temporary activity, in some cases equal in amount to one or two per cent of the activity of the plate, was excited on the walls of the vessel. This activity increased to about three times its original value in the course of thirty minutes after the removal of the active copper, and then decayed regularly to zero.

The amount of this radio-active deposit that can be obtained from a given amount of the direct radium excited seems to be increased by the solution and re-deposit of the emanation X, but it can also be observed from a wire just removed from the radium emanation. If the active wire is placed at once in the testing vessel without having had its temperature raised in any way and removed in a few minutes, an activity about 1/1000 of the whole activity shows itself on the walls of the vessel. The decay of the activity of this deposit is the same as that of the deposit obtained from the active copper. The following table gives the rate of change of the radiation from the walls of a vessel in which an active wire had been left for three minutes after its removal from the emanation:

The active wire retains this power of exciting secondary activity for only a short time after removal from the emanation; after ten minutes the amount it excites is almost inappreciable. Merely Washing the wire in a stream of running water and drying it over a gas flame, as is frequently done to prevent any trace of radium emanation clinging to the wire, increases the amount of the secondary activity to about 1/200 of the whole.

It is evident, then, that some sort of volatile product is given off from the active wire for a time which can excite an activity the rate of decay of which would indicate two changes in the active matter deposited, one producing rays and the other not giving rise to any radiation (E. Rutherford, "Radio-activity," p. 269). It is found that this volatile substance responds to none of the three tests for an emanation, it is not itself radio-active, it cannot pass without sensible loss through material substances such as paper and cotton-wool, and the activity due to it is not concentrated on the negative electrode in an electric field, but distributes itself evenly over all surfaces exposed to it.

The decay of the excited activity from the radium emanation has been explained by Prof. Rutherford on the assumption that there are three changes in the emanation X after its deposit on a solid body. In these three stages one-half the matter is changed in 3 minutes, 21 minutes, and 28 minutes respectively. In the first and third stages the change is accompanied by ionising rave but the second is a rayless change. Now if it be supposed that after the first change has taken place the matter becomes slightly volatile, and some of it is concentrated on surrounding objects a deposit would be obtained which would present the two remaining changes. From the equations for the radio-activity of such a deposit ("Radio-activity," p. 271), it is found that the radiation would increase for about 34 minutes, pass through a maximum, and then decay at the ordinary rate. This is very similar to the behaviour of the deposit obtained in the above experiments.

Curie and Danne (Comptes rendus, March 21) have obtained deposits showing similar characteristics by heating a radio-active wire within a cylinder and measuring the rate of decay of the activity of the cylinder.