Page:Radio-activity.djvu/337

 pure thorium nitrate, no visible deposit was obtained on the anode, but it was, however, found to be radio-active. The activity decayed rapidly, falling to half value in about one hour. Some experiments were also made on the effect of adding metallic salts to thorium solutions and then electrolysing them. Anode and cathode deposits of the oxides or metals obtained in this way were found to be radio-active, but the activity fell to half value in a few minutes. The gases produced by electrolysis were radio-active, but this was due to the presence of the thorium emanation. The explanation of the results obtained by Pegram and von Lerch will be considered later in section 207. It will be shown that the active deposit of thorium contains two distinct substances which have different rates of transformation.

187. Effect of temperature. The activity of a platinum wire which has been exposed in the presence of the thorium emanation is almost completely lost by heating the wire to a white heat. Miss F. Gates found that the activity was not destroyed by the intense heat, but manifested itself on neighbouring bodies. When the active wire was heated electrically in a closed cylinder, the activity was transferred from the wire to the interior surface of the cylinder in unaltered amount. The rate of decay of the activity was not altered by the process. By blowing a current of air through the cylinder during the heating, a part of the active matter was removed from the cylinder. Similar results were found for the excited activity due to radium.

F. von Lerch (loc. cit.) determined the amount of activity removed at different temperatures. The results are shown in the following table for a platinum wire excited by the thorium emanation.

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 * |Temperature| Percentage of |
 * |          |activity removed|
 * Heated 2 minutes     |  800° C.  |        0       |
 * then  "    1/2 minute more| 1020° C.  |       16       |
 * "    "    1/2   "     "  | 1260° C.  |       52       |
 * "    "    1/2   "     "  | 1460° C.  |       99       |