McCisstanp— The Penetrating Radium Rays. 107 
but this divergence from the law disappears in the case of more penetrating 
radiation. 
TABLE B. 
IL, Hales Til. IN 
Platinum, so «| OBA SY = — — 
Merctry, af 053 048 039 036 
ead,  o- ie 056 049 042 037 
Zine, i ae 039 037 034 033 
Aluminium, Ae 038 038 038 — 
Glass, .. At 034 034 0384 084 
Water, .. a 034 "0384 ‘034 034 
This is similar to what Eve (Wature, March 10th, 1904) has found for Rontgen 
rays. ‘The ratio of the coefficient of absorption to the density varies between 
very wide limits when rays from a moderately weak tube are used, the ratio being 
greater for the denser substances. Eve finds that, for gases at any rate, this ratio 
is more nearly a constant when more penetrating Réntgen rays are used. 
Rutherford has determined the coefficient of absorption of y rays for various 
substances (Nature, p. 318, 1902). His numbers are given for comparison for 
substances which occur in the above Table :-— 
Substance. r. Re 
Density. 
Water, oe ae 0338 at ats 0338 
Glass, ae we 086 no Re 085 
Zine, - so OD ge 50. ORB 
Lead, - a ‘770 5% a 068 
Mercury, .. or 920 ye 56 068 
The numbers for water and glass are almost identical with those in the above tables; for lead and 
mercury they are greater even than those in Column I. 
As mentioned at the beginning of the Paper, the fact that the absorption of 
y rays is proportional to the density of the absorbing substance was in favour of 
these rays being charged particles, like other radiation which follows approxi- 
mately the same law. We cannot, however, base much on this, as, according to 
Eve’s work, the same law probably holds for penetrating Réntgen rays; and, 
apart from their absorption, the weight of evidence is certainly in favour of the 
y rays being similar to Réntgen rays. We have rather to consider why charged 
particles travelling with great velocity and Réntgen rays should follow the same 
law of absorption. 
TRANS. ROY. DUB. SOC., N.S., VOL. VIII., PART VIII. dy 
