106 McCiettanp—Zhe Penetrating Radium Rays. 
T being the intensity of the rays after passing through thickness 2, J, the initial 
intensity, and ) the coefficient of absorption. To represent the curves by an 
equation of this type, we must suppose ) to diminish with the thickness of substance 
traversed. These remarks apply chiefly to the curves for zinc, lead, mercury, and 
platinum. The absorption in the case of water, glass, and aluminium is so small 
that sufficient accuracy to show any change in the coefficient of absorption along 
the curves could not be claimed. 
The coefficient of absorption has been calculated for each substance :—I., for 
the first 2°5 mm. of substance traversed; II., for the thickness 2:5 to 5 mm. ; 
III., for the part 5 to 10 mm.; IV., 10 to 15mm. The results are shown in 
Table A. 
In this Table, for the reason given above, no attempt is made to detect any 
change in the coefficient for water, glass, and aluminium. It will be observed 
Tassie A. 
Th, II | III. | IV 
Platinum, O° 1:167 —_— — =< 
Mercury, 50 726 661 "538 “493 
Thead, .. 0 °641 563 “480 "440 
THN, 50 a 282 266 248 236 
Aluminium, .. 104 104 104 — 
Glass, .. aC 087 087 087 087 
Water, .. oC 034 034 034 034 
that for the heavier substances there is a rapid decrease of the coefficient of 
absorption as greater distances of the substance are traversed by the rays; no such 
marked change takes place for the lighter substances. 
This points to the conclusion that the y rays are somewhat heterogeneous, and 
that the difference between the coefficients of absorption for the more penetrating 
and the less penetrating parts is more marked the greater the density of the 
substance used to absorb the rays. 
Table B gives the result of dividing the coefficient of absorption by the 
density of the substance. It will be seen that in column I. the numbers vary 
considerably, the law that the coefficient of absorption is proportional to the 
density not being followed at all closely. The agreement is, however, much 
closer in succeeding columns until in column IV. it is very close indeed. For the 
less penetrating radiation, the absorption increases more rapidly than the density; 
