694 Dr. Geiger and Prof. Rutherford on the Number of 



then, by a simple integration, it can be shown that the 

 number n of a particles incident per second on the area A is 

 given by 



2 V s/a* + dV 



A simple example will serve to illustrate the method of 

 calculation. The uranium film No. 1 (see table later) con- 

 tained 10'43 milligrams of uranium oxide (U 3 8 ) spread on 

 an area of 5*9 square cms. 515 scintillations were counted, 

 and the average number of scintillations observed corre- 

 sponded to 5*16 per minute, and per second *086. Making 

 the 8 per cent, correction for the imperfection of the screen, 

 the corrected value becomes *093. This is the value of n to 

 be substituted in the formula. 



A = 3*1G sq. mms. d = 2*06 cms. a = 1*37 cms. 



Substituting these values in the formula, 



a = 35-0. 



Now the weight of film per square centimetre was 

 1*77 mg. U 3 8 , or 1*50 mg. uranium. Consequently, from 

 this experiment, the total number of a particles emitted per 

 second per gram of uranium is 2*33 XlO 4 . 



The chief difficulty of the experiments lay in counting 

 accurately a sufficiently large number of scintillations. The 

 number cf scintillations observed in the microscope varied 

 from one to five per minute in the case of uranium or 

 thorium. While different observers agreed closely in 

 counting scintillations due to radium or polonium when 

 30 to 50 scintillations were seen per minute, the agreement 

 was not so good for uranium films. This difference is in 

 part clue to the fact that the eye becomes quickly fatigued 

 when only a few scintillations appear on the screen per 

 minute. This was especially marked in counting the scintil- 

 lations from uranium, which are relatively much fainter than 

 those from radium 0. In the case of uranium and thorium 

 minerals, where the scintillations are on the average much 

 brighter than those from uranium, the counting was relatively 

 easy. The brightness of scintillations of course depends on 

 the range of the a particle striking the screen. We shall 

 see later that the range of the a. particle, and consequently 

 the intensity of the scintillations from uranium, is less than 

 from any other radioactive substance. 



The active materials used in these investigations were 



