598 Dr. N. Bohr on the Decrease of 



columns I. and III. The values in these columns are calcu- 

 lated by putting logr = 8*75 (see above) and logr = 4-44 

 (see below) respectively. If instead of log £0 = 5*37 we i ]ac [ 

 used one of the latter values, we should instead of 14*4 have 

 to put 72 = 8*1 or n — 22' 5 respectively, in order to obtain the 

 observed value for x x . It will therefore be seen that the 

 considerable difference between the values in the columns I., 

 II., and III. offers a method of determining n, even in cases 



where -2 log v is not known beforehand. 



Marsden and. Taylor could not observe any a particle with 

 a velocity smaller than 0*42 V . When the velocity had 

 decreased to this value the particles apparently disappeared 

 suddenly. This peculiar effect is in striking contrast to what 

 should be expected on the theory. It appears, however, that 

 it may possibly be explained by a statistical effect due to a 

 small want of homogeneity in the a-ray pencils used. In the 

 first part of the velocity curve the slope varies gradually, and 

 a possible small want of homogeneity will have only a very 

 small effect on the mean value of the velocity. But near 

 the end of the range the slope of the curve is very steep, 

 and if the pencil for some reason is not quite homogeneous, 

 the effect will be that, as we recede from the source, more 

 and more of the particles will so to speak suddenly fall out 

 of the beam. In this way the velocity will not start to 

 decrease rapidly until almost all the particles are stopped ; 

 but then the beam will contain so few particles that the final 

 descent may be very difficult to detect. 



The values in column V. correspond to Marsden and 

 Taylor's results for the velocity curve of rays from radium 

 in aluminium. The value for x x corresponding to V = |V a 

 was 9*64 . 10 ~ 3 , measured in gr. per cm. 2 The value for K x 

 in aluminium if x is measured in gr. per cm. 2 is 9*81 . 10 36 * 

 If for aluminium we assume rc = 13, and in (20) intro- 

 duce ^ = 9-64. 10 3 for V = ^V , we get Xogz^ — i.'^ and 



-% log v = 39'02. As mentioned above this corresponds to 



the values in column III. It will be seen that the values in 

 column V are much closer to those in III. than to those in 

 I. and II., but the agreement is not nearly so good as for 

 air. This may partly be due to the difficulty in obtaining 

 homogeneous aluminium sheets, but it may also be due to 

 the fact that the assumptions underlying the calculations 

 cannot be expected to be strictly fulfilled for all the electrons 

 in the aluminium atom (see page 586). For elements of 



