Transformation of the Radioactive Emanations. 621 



possible explanation of the reason why no substance has been 

 found emitting a rays of range shorter than 2'58 cm. For 

 example, the life of a substance which emitted a, rays of 

 range 1 cm. would be so long, and consequently its trans- 

 formation so slow, that its activity would be beyond the limits 

 of detection by present methods. 



Experiments are at present in progress with the view of 

 determining with accuracy the ranges of the products which 

 are yet uncertain. The result of such a complete investi- 

 gation may I e expected to show definitely whether the relation 

 given holds generally for all the substances emittting « rays. 



We are indebted to Prof. Rutherford for his help and his 

 interest in these experiments. 



The Victoria University, Manchester, 

 Physical Laboratories. 



LVII.I. Transformation and Nomenclature of the Radioactive 

 Emanations. By Prof. E. Rctherford, E.R.S., and 

 Dr. H. Geiger, University of Manchester *. 



IN a recent paper, H. Geiger* has described experiments 

 which show that the emanation of actinium contains a 

 product ot very quick transformation, which emits a rays of 

 long range, 6'5 cm. in air, while the range of the a particles 

 from the emanation itself is 5' 7 cm. Immediately after its 

 formation, this new substance has a positive charge, and 

 travels to the negative electrode in an electric field. By 

 assuming that this positive currier has the sume mobility in 

 air as a positive ion produced in air, it was deduced that the 

 quick product was half transformed in about 1/500 of a 

 second. The proof of the existence of this product at 

 once explained the observation made some time before 

 by Geiger and Marsden that the emanation of actinium 

 apparently emitted two a ray particles at nearly the same 

 time. Since the new product is almost completely trans- 

 formed in 1/50 of a second after its formation, the a ray 

 particle from the emanation itself would be followed within 

 this interval by one from the new product, and the interval 

 between them could not be detected by the eye using the 

 scintillation method. In a previous paper, Geiger and 

 Marsden f have also shown that the emanation of thorium 

 emits two a particles in rapid succession. In this case there 

 was on the average a distinct interval between the appearance 

 of two scintillations on the zinc sulphide screen, indicating 



* Communicated bv the Authors. 



t Phil. Mag. July 1911. % Phys. Zeit. xi. p. 7 (1910). 



