﻿440 Dr. H. Geiffer and Mr. J. M. Nuttall on the 



o 



from each of the subsequent products, and this has been con- 

 firmed by the scintillation method by Geiger and Rutherford*. 

 This may be ascribed to the existence of two successive 

 as-ray products, or to the simultaneous emission of two 

 a particles in the disintegration of each atom. The latter 

 hypothesis is excluded by the experiments of Marsden and 

 Barratt f, who found no evidence that two a. particles were 

 emitted simultaneously. It seems therefore probable that 

 there exist two successive products each of long period which 

 cannot be separated by ordinary chemical methods. To settle 

 definitely whether this is the case, experiments have been 

 made which will be described in this paper, to determine 

 accurately the ionization curve of the a particles from a very 

 thin film of uranium. For purpose of comparison, curves 

 were taken under exactly the same conditions for films of 

 ionium and polonium. Assuming that there are present in 

 uranium two a-ray products whose difference of range is at 

 least one millimetre (which means that the two periods differ 

 by about a factor 10), this would affect the ionization curve 

 to such an extent that it would be distinguishable from the 

 ionization curves of simple a-ray products like ionium or 

 polonium. 



The method of determining the ionization curve of an a 

 particle, originally devised by Bragg J , consists in the measure- 

 ment of the ionization produced by a parallel pencil of a rays 

 in a shallow ionization vessel placed at different distances 

 from the source. This method can easily be applied if the 

 material can be obtained as a very thin film of high activity. 

 Since, however, uranium shows only a very low activity it 

 was necessary to modify the original arrangement of Bragg. 

 In the first place, it seemed advisable to make the dimensions 

 of the apparatus as large as possible to enable the use of films 

 of very large area. In the second place, it appeared advan- 

 tageous to combine the ionization vessel and the measuring- 

 instrument by using an electroscope which has a small 

 capacity and allows the accurate measurement of very minute 

 ionization currents. 



The arrangement used in the experiments will be seen 

 from fig. 1. The main part of the apparatus consisted of two 

 large bell-jars AA and BB, separated by a brass plate (JO. 

 A large number of holes — about 250 — each 3'7 mm. in 

 diameter and 8 mm. high, forming a circular "grid"" of 



* H. Geiger and E. Rutherford, Phil. Mag. vol. xx. p. 691 (1910). 

 t K Marsden and T. Barratt, Proc. Pliys. Soc. vol. xxiii. p. 3G7 (1911 ). 

 X W. H. Bragg and R. 1). Kleeman, Phil. Mag. vol. viii. p. 726 . 



(190-1) ; vol. x. p. 318 (1005). 



