914 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
be the same for all substances, assuming (1) that the absorption of y rays de- 
pends only on the density of the material traversed and not on its atomic 
weight; (2) that the 6 rays behave like the y rays in this respect; (3) that 
the y rays are not liable to selective absorption. 
He then shows by experiment that the “ emergence” radiations show no sign 
of following the B ray law, as the incidence radiations do; that they are of the 
same order for all substances, and that the observed differences promise to be 
readily explained when proper account is taken of the imperfections of each 
of the three assumptions mentioned above. 
He further discusses the possibility that the cathode particle of the X-ray 
tube may become an X ray by picking up a positive of small mass at the anode, 
and may afterwards by dropping the positive become the secondary cathode 
ray, the speed remaining approximately the same throughout. 
