﻿. Straggling of a. Particles by Matter. 47 



rough indication of the absolute magnitude of the straggling. 

 However, comparative values of the straggling under different 

 conditions should be given fairly accurately by the method 

 adopted. On the other hand, in the counting experiments 

 the assumption is made that the zinc sulphide screens or 

 photographic plates used have the same efficiency for a rays 

 of low speeds as for those of high speed. This assumption 

 is not altogether justifiable. 



§ 3. Straggling in Gases other than Air. 



The ionization curve in hydrogen was determined with 

 the same apparatus already used for air. The gas was 

 obtained from a cylinder of compressed hydrogen stated 

 by the makers to be of more than 98 per cent, purity. 

 Small impurities are unlikely to affect the straggling- 

 mate rially. 



It was again found that a considerable portion of the eud 

 of the ionization curve could be represented by a straight 

 line. When the range of the a. particles was reduced so as to 

 give the same range as in air, the projection of the straight 

 line was 2*05 mm. with a probable error of 3 per cent. 



The straggling in air and hydrogen may be deduced from 

 the ionization curves given by other observers. The results 

 agree in every case within the limits of error, although the 

 conditions for accuracy were less favourable than in the 

 present experiments. The value 2'0 mm. for polonium in 

 hydrogen may be obtained from some ionization curves given 

 by Taylor *. From the results of Lawson (loc. cit.) for 

 polonium the straggling was determined as 3"1 mm. in air 

 and 2'2 mm. in hydrogen. 



The straggling in oxygen has also been deduced from 

 experiments made in that gas with the present apparatus, 

 using ThC. The value found was 3*36 mm., when the range 

 was increased to the same value as in air. 



The straggling in several other gases may be deduced 

 from the ionization curves given by Taylor (loc. cit.), although 

 the error involved is probably of the order of 10 per cent. 

 The collected results of stragglingin gases are given in fig. 2, 

 which shows the straggling plotted against molecular stopping 

 power. The values plotted for air, hydrogen, and oxygen 

 are from the writer's results; the remainder are taken from 

 es. 



* Taylor, Phil. Mag. xxi. p. 571 (1911). 



