a Particles emitted by Uranium and Thorium. 693 



Arrangement of Experiment. 

 The scintillation method was adopted in order to count 

 the number of a, particles from a known weight of active 

 material. A small quantity of the material under examina- 

 tion was finely powdered in an agate mortar, and then mixed 

 with alcohol or ether and deposited as a thin uniform film on 

 a thin sheet of aluminium or glass. The method adopted 

 was similar to that first used by McCoy. Care was taken 

 that the powder suspended in the liquid was well stirred in 

 order to avoid a separation of the lighter from the denser 

 portions. The weight of the active film was determined by 

 weighing the plate before and after the active material had 

 been removed. It was desirable to use very thin films in 

 order that all the a particles might emerge without much 

 loss of their range. In the case of uranium, however, the 

 number of a particles emitted was so small that they were 

 difficult to count with accuracy. For this reason thicker 

 films were in some cases purposely employed. The efficiency 

 of the zinc sulphide screen was tested by counting the number 

 of a particles emitted from a definite quantity of radium C. 

 The number of scintillations observed was found to be 8 per 

 cent, less than the actual number of a particles incident on 

 the screen. The latter value was calculated from the known 

 result that one gram of radium and each of its products 

 emits 3*4 x 10 10 a particles per second. In the initial experi- 

 ments the number of scintillations was counted by placing 

 the screen close to the active material. In this case, the 

 number of a particles striking the screen is equal to one half 

 the total number emitted from an area of the active film 

 equal to the area of screen seen in the microscope. This 

 method is open to some objections, for it requires that the 

 film should be very uniformly spread and, in addition, very 

 thin, for otherwise the particles emitted at an oblique angle 

 suffer a considerable loss of range in the active material 

 itself. The lack of .uniformity of the film can be corrected 

 for by counting at different points parts of the film, but this 

 involves much labour. 



In most of the experiments the active matter was spread 

 in a circular area, and the small zinc sulphide screen was 

 placed parallel to the film and opposite to its centre. 



If a = radius of circular film, 



d = distance of screen from centre of film, 

 A = area of screen observed in field of microscope, 

 cr = total number of particles emitted per second 

 per square centimetre of surface of film, 





