tion — would be the same far all materials. There should be 

 no such relation between the amount of the radiation and the 

 atomic weight of the plate, as various observers have shown 

 to be true for the secondary cathode radiation of "incidence," 

 a relation which is closely parallel to that found in the case 

 of )8 rays. 



Experiment is in agreement with this theory, for it shows 

 that no such relation exists in respect to the emergence radia- 

 tions ; in marked contrast to what happens in the case of 

 the radiations from the front sides of plates of various mate- 

 rials, the incidence radiations. 



It is true that the emergence radiations are not all equal, 

 but this is to be expected, because (1) the amount of second- 

 ary cathode radiation depends, as Kleeman has shown, on the 

 previous screening of the y rays ; (2) the )8 rays are not ab- 

 sorbed strictly according to a density law ; (3) the y rays 

 also depart from this law. We have made no serious attempt 

 as yet to disentangle the effects of these various disturbing 

 factors. In fact, the task promises to be long and intricate, 

 for it will be necessary to find out how much of the ioniza- 

 tion in the chamber is due to each class of rays : to discover 

 the law of distribution of the radiations in space, so that the 

 form of the chamber may be allowed for, if necessary ; to 

 find out the nature of the departures from the density law 

 of those j8 and y rays which are in question : and so on. 

 Nevertheless, the results are satisfactory, so far as we have 

 gone. The amount of emergence radiation is found to depend 

 on the previous screening of the rays. In one case the in- 

 version of a C, Pb pair of plates from C, Pb to Pb, C, altered 

 the current in the ratio l.l'll when the rays had been pre- 

 viously screened by Pb : but in the ratio 1 : "96 when the screen 

 was changed to C. Again, when the rays had previously pass- 

 ed through an iron screen, the inversion Pb Fe to Fe 

 Pb changed the current in the proportion 1:1'12, but when 

 a lead screen was substituted for an iron one the change was 

 1:1*04. In illustration of the effect of the second disturb- 

 ing factor mentioned above, we have found that, other things 

 being equal, the substances of small atomic weight give the 

 most secondary radiation, in a general way ; and it may be 

 no coincidence that in some cases we have found Sn and Fe 

 to give surprisingly small amounts. This is in agreement 

 with what is to be expected, for it is clear, on consideration 

 of the argument already given, that the greater the )8 ray 

 absorption of a substance in proportion to its density, the less 

 ''emergence" radiation should issue from it. Some observers 

 have found Sn and Fe to possess exceptional absorbing pow- 

 ers. We do not wish, however, to lay any stress upon these 



