=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[284] 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[285]. Temperature Percentage of activity removed Heated 2 minutes 800° C. 0 then „ ½ minute 1020° C. 16 more „ „ ½ „ 1260° C. 52 „ „ „ ½ „ 1460° C. 99 „ The effect of heat on the volatilization of the active deposit of radium has been examined in detail by Curie and Danne. The interesting and important results obtained by them will be discussed in chapter XI, section 226. =188. Effect of variation of E.M.F. on amount of excited activity from thorium.= It has been shown that the excited activity is confined to the cathode in a strong electric field. In weaker fields the activity is divided between the cathode and the walls of the vessel. This was tested in an apparatus[286] shown in Fig. 70. [Illustration: Fig. 70.] _A_ is a cylindrical vessel of 5·5 cms. diameter, _B_ the negative electrode passing through insulating ends _C_, _D_. For a potential difference of 50 volts, most of the excited activity was deposited on the electrode _B_. For about 3 volts, half of the total excited activity was produced on the rod _B_, and half on the walls of the vessel. Whatever the voltage applied, the sum of the activities on the central rod and the walls of the cylinder was found to be a constant when a steady state was reached. When no voltage was applied, diffusion alone was operative, and in that case about 13 per cent. of the total activity was on the rod _B_. The application of an electric field has thus no influence on the sum total of excited activity, but merely controls the proportion concentrated on the negative electrode.