692 Mr. E. Marsden and Dr. P. B. Perkins on the 



the source and screen so that a smaller distance between the 

 two could be used. The arrangement is shown in fig. 1. 



Fig:, l. 



To PUMP 

 AND MANOMETER 



The a particles from the source A passed through a sheet 

 o£ mica B and impinged on the zinc-sulphide screen Z, where 

 the scintillations were observed by a microscope M. The 

 distance of A to Z could be varied in different experiments, 

 and was generally between 09 and 1*5 cm. The thickness 

 of mica was also varied in different experiments between 

 5 and 7 cm. air equivalent The pressure of air in the apparatus 

 was measured by a manometer, and thus the equivalence of the 

 distance AZ could be expressed in terms of air at 15° C. and 

 76 cm. By adding the equivalent of the mica the ; * range " at 

 which the a particles were counted could thus be determined. 



The procedure in an experiment was generally as follows: — 

 A was exposed for several hours to a source of actinium 

 emanation and then placed in a convenient position inside the 

 apparatus with a sheet of mica of known thickness place 1 

 over it. Countings of the scintillations were then made at 

 different air-pressures for about 40 minutes, in which time 

 the source had decayed to less than half value. A was then 

 removed and the total number of « particles emitted measured 

 by counting the scintillations on a zinc-sulphide screen placed 

 at a known distance in a completely exhausted apparatus. 

 From the known decay of the actinium active deposit all the 

 measurements could be corrected to the initial time, and thus 

 the percentage of particles reaching any ic range" could be 

 determined. The curve marked "Actinium " in fig. 2 gives 

 the results of a particular experiment. The other curves give 

 the results of exactly similar experiments with radium C and 

 thorium C 2 respectively; in the latter case account has been 



