472 Prof. Bnmstead and Mr. McGougan on Emission of 



consisting partly of electrons moving with considerably higher 

 velocities than the hitherto recognized S-rays. An in- 

 vestigation of this absorbable radiation will be described in 

 the next section. 



Experiments similar to those which have been described at 

 length in the case of aluminium were made also with copper, 

 gold, lead, and platinum. In all cases the surfaces of the 

 metals were made clean and bright by fine sandpaper. The 

 magnitude of the 8 -ray currents obtained from the various 

 metals under similar conditions were not very different from 

 each other, when correction was made for the decay of the 

 polonium in the intervals between the experiments. The 

 main purpose of the present investigation was to ascertain 

 the variations in the form of the ionization curves for 

 different metals and not their absolute magnitudes; for this 

 reason no attempt was made to get an accurate determination 

 as to the latter point. It is rendered difficult by the fact, 

 discussed in § 1, that the 8-ray current falls off with the 

 lapse of time after the liquid air has been applied to the 

 cbarcoal. This effect was observed in all the metals studied, 

 but it was not quite so marked as in the case of the thin 

 platinum foil described in § 1. However, the diminution in 

 the current sometimes amounted to as much as 20 per cent, 

 and continued to be noticeable for two or three daj^s *. As 

 in the case of the platinum foil, the relative values at 

 different points of the range of the a-particles were not 

 affected by this variation, the ratio of the ionizations at 

 any two points remaining practically constant. 



The results for the different metals are given in Table III. 

 and plotted in fig. 3. The values used are the currents due 

 to the S-electrons alone, corresponding to Curve 1.5 in fig. 2. 

 In order to make the comparison easier they have been 

 reduced to the same scale by making the current for one 

 thick foil the same for all the metals ; the other currents are 

 then altered in the same ratio. 



The points for the different metals lie so closely together 

 that only one curve has been drawn. Anyone who has had 

 experience with such measurements will recognize that the 

 differences observed are too small to have any significance. 

 Even in the case of aluminium, which appears to differ some- 

 what from the others, the differences are not at most more 

 than 2 or 3 per cent. ; and differences of this order, which 

 are obviously accidental, occur in all the curves. We are 



* Our observations in regard to the variations in the magnitude of the 

 6-ray current with different metals is in substantial agreement with those 

 of Campbell, Phil. Mag\ xxi. p. 298 (1011 ). 



