25 



Uran 



lum. 



The uranium was used in the form of the green oxide, 

 UaOy, and was freed for the time from Uranium X. This was 

 not necessary, but convenient, as it diminished the fi ray 

 correction. The uranium was ground to a fine powder, and 

 placed in a shallow depression turned in a metal plate, the 

 diameter of the recess being 3'17 cm. and the depth 1-32 inch, 

 which was far more than enough to make sure that the a 

 rays from the lowest stratum could not get out. The surface 

 of the material was carefully smoothed by the aid of a polished 

 metal plate. A potential of 300 volts was used, which was 

 nearly sufficient to saturate ; more was not necessary, as only 

 relative ionisations were in question. Aluminium foil w^as 

 used as the absorbing layer, the weight and area of each 

 piece being measured so as to obtain the product of the 

 density p and the thickness d. In the following table 



the first column gives the value of pd of the foil used, and 

 the second the corresponding current, being the mean of five 

 readings of the leak for ten seconds. 



I. II. III. IV. V. 



The last line shows that wnen two layers of tinfoil were added 

 to the aluminium foil already covering the uranium the leak 

 was reduced to 34. Each layer of foil was equivalent to 

 about 17 mm. of air, and the aluminium to about 21, so that 

 the whole cut off the a rays completely, for their range was 

 know^n to be not more than 3*5 cm. This leak of 34 was 

 therefore due to (B rays, and the normal leak of the appara- 

 tus. The third column shows the result of subtracting 34 

 from all the figures of the second column, and reducing to a 

 decimal fraction of I (the maximum current). The num- 

 bers so obtained were then considered as so many ordinates of 

 the thick layer curve A ; and the corresponding abscissae found 



