282 EXCITED RADIO-ACTIVITY [CH. 
formed. With weaker fields, a portion diffuses to the sides of the 
vessel, and produces excited activity on the positive electrode. 
183. Effect of pressure on distribution of excited 
activity. In a strong electric field, the amount of excited activity 
produced on the cathode is independent of the pressure down to a 
pressure of about 10 mms. of mercury. In some experiments made 
by the writer?, the emanating thorium compound was placed 
inside a closed cylinder about 4.cms. in diameter, through which 
passed an insulated central rod. The central rod was connected to 
the negative pole of a battery of 50 volts. When the pressure was 
reduced below 10 mms. of mercury, the amount of excited activity 
produced on the negative electrode diminished, and was a very 
small fraction of its original value at a pressure of ~;mm. Some 
excited activity was in this case found to be distributed over the 
interior surface of the cylinder. It may thus be concluded that at 
low pressures the excited activity appears on both anode and 
cathode, even in a strong electric field. 
Curie and Debierne® observed that, if a vessel containing an 
emanating radium compound was kept pumped down to a low 
pressure, the amount of excited activity produced on the vessel 
was much reduced. In this case the emanation given off by the 
radium was removed by the pump with the other gases con- 
tinuously evolved from the radium compound. On account of the 
very slow decay of activity of the emanation, the amount of excited 
activity produced on the walls of the vessel, in the passage of the. 
emanation through it, was only a minute fraction of the amount 
produced when all the emanation given off was not allowed to 
eSCa]} ye, 
184. Transmission of excited activity. The characteristic 
property of excited radio-activity is that it can be confined to the 
cathode im a strong electric field. Since the activity is due to a 
deposit of radio-active matter on the electrified surface, the matter 
must be transported by positively charged carners. The experi- 
ments of Fehrle* showed that the carriers of excited activity travel 
1 Phil. Mag. Feb. 1900. 2 ¢. R. 132, p. 768, 1901. 
3 Phys. Zeit. 3, No. 7, p. 130, 1902. 
