638 Radio-active Products of Short Life. 



atom in reaching the rotating disk is limited by the dielectric 

 strength of the air, the velocity * in an electric field of one 

 volt per centimetre being about 1*4 cm. per second. The 

 minimum time lost will therefore be about 10 ~ 5 second. 



The time lost in reaching the measuring instrument will 

 be least when the product emits a particles, and these are 

 counted by the method of scintillations. Since the distance 

 between source and zinc-sulphide screen need be not more 

 than a few millimetres, measurements may be taken between 

 10 " 5 and 10 ~ 4 second after the separation of the product. 



It is intended by this method to extend the investigation 

 into the possible complexity of radium C and of thorium G 

 (period 55 min.) ; preliminary experiments, in which the 

 loss of time was much greater, having given negative results. 



It is evident, however, that, for a product of which the 

 parent does not emit a particles, the main difficulty will be 

 in effecting the separation. 



It is intended also to use the method for studying the 

 nature of the /3 radiation from radium C 2 t, the advantage 

 being obvious of working with a source which decavs to 

 half value with the period of radium C, 19*5 minutes, rather 

 than with that of radium C 2 itself, viz. only 1*4 minutes. 



By increasing the size of the ionization vessel, more than 

 half of the activity of the separated product will be available 

 for measurement. 



Summary. 



1. Actinium emanation is succeeded by a solid product, 

 which emits a particles, and has a half-value period '0020 

 second. 



2. Thorium emanation is succeeded by a similar product 

 with half- value period 0*14 second. 



3. The product obtained by recoil from actinium active 

 deposit is pure Act D. The supposed complex nature of 

 Act C is therefore not confirmed. 



In conclusion we wish to express our sincere gratitude to 

 Professor Rutherford, both for the loan of much radioactive 

 material, and for the kind interest which he has taken in 

 this research. 



Physical Laboratoiy, 

 University of Manchester. 



* Franck, Verhand. d. D. Phys. Gesel. vol. xi. p. 55 (1909). 

 t Fajans, Plujs. Zeit. vol. x. p. 372 (1911). 



