the a. Particles of Radium. 325 



residual errors o£ the experiment become relatively small. In 

 particular, consecutive readings of the same leak become 

 much more nearly in agreement than before ; when the 

 readings are large, the difference between two such readings 

 is generally less than one per cent, of the whole. For small 

 readings it is proportionately greater ; and it is quite clear 

 that much of this irregularity is due to some extraneous 

 cause, and is not due to variability of the principal leak, or 

 inaccuracy either of the observer or his apparatus. Other 

 experimenters have made a similar observation. 



In the case of all the measurements recorded in this paper, 

 the current was allowed to run into the electrometer for ten 

 seconds ; the mean of the first and second points of rest of 

 the needle was taken as the correct reading. This was 

 justifiable, as relative, not absolute, measurements were 

 wanted ; and as the difference between these two mea- 

 surements was very nearly proportional to the whole 

 deflexion. 



Carves B, C, and D in fig. 1 show the state of the radium 

 at definite intervals after preparation of the layer. Similar 

 curves were published in our previous paper, but the curves 

 now shown are much more accurate, and supply much more 

 information. In the first place, the division of the a rays 

 into four groups is now seen very clearly, and the range of 

 each group can be measured accurately. In the former 

 curves, the separation of the two central groups was not well 

 effected, but now it is plain ; the improvement being mainly 

 due to the greater thinness and evenness of the radium 

 layer. 



In the same figure, curve B shows the state of the radium 

 when 28 days old. According to Rutherford's theory the 

 number of « particles in each group is the same, provided the 

 radium preparation is old enough to be in radioactive equili- 

 brium. This can be proved from the curve E in the following 

 manner. 



The curved side ab, belonging to Ra C, is produced to c, as 

 shown. The line d e represents a certain ionization produced 

 by /3 and y rays which is intercepted by the aluminium screen. 

 It is the small difference between the readings at (say) 

 7*5 cm. when the aluminium is on and off. When the radium 

 is within range, and the ionization of the a rays is measured 

 as the difference between the on and off readings, this small 

 quantity of /3 and 7 ionization is included in the difference. 

 Its value must be nearly constant over all ranges, as it is a 

 certain fraction of the total /3 and 7 ionization, which latter 

 is found to be constant by experiment. We may therefore 



