ORIGIN OF ACTINIUM 137 



and having a period of only 1-65 minutes. The 

 possibility that actinium was produced in a /3-ray 

 change from an isotope of radium was experimentally 

 disproved, and there remained only the second 

 alternative, which was rendered the more probable 

 by the existence of a member, uranium- K, dis- 

 covered by Antonoff, isotopic with uranium-A'i, 

 and simultaneously produced with it from uranium 

 in relative quantity such as is to be expected, if 

 it were the first member of the actinium series. 

 Uranium- K, like uranium-^, gives soft /?-rays, 

 and hence its unknown product must be the isotope 

 of uranium-^,, and might also well prove to be 

 the unknown direct parent of actinium in an a-ray 

 change of long period. 



During the year the missing element has been 

 found in two independent investigations (Soddy and 

 Cranston, Proc. Roy. Soc., 1918, [A\, 94, 384; 

 O. Hahn and L. Meitner, Physikal. Zeitsch., 1918, 

 19, 208). The problem as it presented itself to us 

 was so to treat a uranium mineral as to separate an 

 element, if present, which possessed the chemical 

 character of the known but hopelessly short-lived 

 uranium-^* using the latter as an indicator in trying 

 possible methods beforehand. The method adopted, 

 distillation at an incipient red heat in a current of 

 carbon tetrachloride vapour and air, was found to 

 be very effective in volatilising uranium-^ from 

 uranium-^!, and when applied to pitchblende it was 

 found to give a product in which none of the known 

 pre-emanation members of the disintegration series 

 were present. Thus was obtained a preparation 

 from which actinium was at first absent, but which, 

 with the lapse of time, continuously generated 

 actinium, as characterised beyond the possibility of 

 doubt by means of its active deposit. 



It should be mentioned that the exact point at 



