42 



weight, weight for weight of screen ; or, in the usual phrase- 

 ology, since Kj p is smaller for light atoms than for heavy 

 atoms, we expected the emergence radiations of C and Al to 

 be much greater than those of, say, W and Pb. Fig. 2 shows 

 that this expectation was realized ; the results are shown gra- 

 phically along the curve AA. 



The representative points do not lie closely on a smooth 

 curve. To some extent, no doubt, this is due to experimental 

 error, for the measured quantities are very small. But we 

 are inclined to think that the departures from regularity are 

 to some extent real. We hope to settle this point partly by 

 improving our apparatus, and so attaining greater accuracy 

 of measurement, and partly by obtaining better knowledge of 

 the corrections to be applied to the observations. For our 

 present purpose, it is enough to draw a smooth curve such as 

 A A, and to ignore the variations from it. 



The figure shows a rapid decrease in the emergence radia- 

 tions as we proceed from C to Te. There is then a small rise 

 to W and Pb. This is easily explained, and exemplifies a 

 very important feature of the problem. We must remember, 

 not only that A//) is fairly constant for the heavy atoms, but 

 also that the lead screen did not completely remove the soft 

 rays ; in other words, that the pencil of y rays was not quite 

 homogeneous. 



This will be clearly understood by comparing the curve 

 A A with the curve BB, which represents the results we ob- 

 tained when the lead plug was removed and the y rays had 

 passed through only the wall of the vessel containing the 

 radium before entering the chamber. There was therefore a 

 quantity of soft radiation in the stream of y rays ; the effect 

 was to increase considerably the emergence radiation of Pb 

 and W relatively to that of the other substances. This was 

 as it should have been. We know that hard y rays pay no 

 attention to atomic structure, but that soft rays distinguish 

 between atoms of different weight. In the table given by 

 Wigger (Jahrbuch der Radioaktivitat, 1905, p. 432) the values 

 of Xj p for thin sheets of Pb and Zn are '068 and *039 re- 

 spectively ; but when the rays have been hardened by passing 

 through 2'8 cm. of lead, each co-efficient has become nearly 

 "02. Thfls, the hard rays treat lead and zinc alike, but softer 

 rays are more absorbed by the former than the latter, weight 

 for weight. When a stream of y rays is passed through a 

 lead plate, the soft rays are rapidly converted into )3 rays. 

 Consequently, a thin lead plate produces a large quantity of 

 emergence radiation, due principally to the slow /3 rays pro- 

 duced by the soft y rays. The effect is further discussed 

 later on in the paper. But this is rapidly used up ; conse- 



