[KING] ABSORPTION PROBLEMS IN RADIOACTIVITY 103 
TABLE II. 
| 
Z Aa In =SSoilll @mn, |) Ja == ell Gan.) In NT nr 
metres. kh = -O1 a = ol = nf) TIENNE 
0 0 1-000 1-000 1-000 | 1-000 
aot be -05 -472 -706 -812 | -828 
DD -] -340 - 535 -696 -722 
45-5 -2 239 -319 - 543 -574 
67-2 -3 -178 -288 -437 -469 
114 -5 -109 -182 -297 -327 
159 -7 -078 -123 -214 -235 
227 1-0 -050 -073 -131 -148 
The results are shown graphically in Plate II. These curves show 
very distinctly that even in the case of distribution of radioactive 
matter throughout an infinite thickness, practically the whole effect 
on the gradient is due to a layer very little more than 11 cms. thick. 
This point is of some importance in an actual measurement of the 
gradient. Unless the penetrating radiation is measured over a fairly 
level and homogeneous area, we cannot expect (10) to give an adequate 
representation of the gradient.! 
Fi. 3. 
4. It is interesting to notice that a considerable number of absorp- 
tion problems for flat plates involve the exponential integral. Several 
such problems are considered by Soddy.? 
The results (due to Sir Joseph Larmor) are given for a point-source 
of radium in contact with a plate of thickness h. In the notation of the 
present paper the ratio of the total radiation over an angle of 180° on 
1 Cf. Wulf, Phys. Zeit., Sept. 15, 1910, p. 811. 
2 Soddy, (F. and W. M.) and Russell, “The Question of the Homogeneity of y 
Rays” Phil. Mag. XIX, May, 1910, p.725. 
