of the Actinium Emanation. 203 



the actinium emanation, namely 5*8 cm., is in good agree- 

 ment with that given above. 



The distinct difference in range of the a particles from the 

 actinium emanation makes it highly suggestive that the 

 emanation consists of two successive products, which both 

 emit a rays. The 4 second product — the actual emanation — 

 emits a rays of range 5*7 cm. and the following short-lived 

 product rays of a range of 6*5 cm. 



An experiment which gives a still more direct evidence of 

 the existence of this product, and also allows an approximate 

 determination of its period, was suggested to me by Pro- 

 fessor Rutherford, for which I am greatly indebted to him. 

 The arrangement is shortly this. Actinium emanation was 

 sucked through a metal tube of rectangular cross-section, as 

 shown in fig. 2. The part AB of the tube, about 2*5 mm. 

 wide and 1 cm. long, was cut out and the opening 



Fig- 2. covered by a thin sheet of mica. A wire W, 

 insulated from the tube, and placed out of centre, 

 as seen in the figure, could be charged negatively 

 to any potential required. 



If the short-lived product following the 

 emanation is charged — as may reasonably be 

 expected from an analogy of the corresponding products in 

 the radium series — this product will be drawn to the wire W 

 with a speed depending on the intensity of the electric field, 

 as soon as it is formed. The screen, on which the scintilla- 

 tions were observed, was placed at such a distance from the 

 mica window that only the a particles of longest range could 

 strike it. Any radiation from the wire W was screened off 

 by the box. 



Countings were taken alternately with and without the 

 electric field. A very marked difference was found. The 

 number of scintillations decreased to about £ for a potential 

 difference of about 30 volts, and to about I for a difference 

 of 200 volts. More intense fields did not produce any in- 

 creased effect, and this was probably due to the fact that 

 some of the atoms of the short-lived product lost their charge, 

 either by recombination or by contact with the walls, before 

 the field had pulled them to the wire. 



Since the migration constant of the radioactive atoms is 

 the same as that of positive ions *, it could be calculated that 

 the period of the new substance is of the order of 5 J - second. 

 This value can of course only be considered as a rough 

 estimate ; experiments by another method are at present in 



* J. Franck, Verhandlgn. d. d. Phys. Ges. xi. p. 397 (1909). 



