McCuetianp and Hackerr—Zhe Absorption of B Radium Rays by Matter. 49 
The following table gives asummary of the results. As described in the case of 
zinc, the coefficient \’ was determined from plates of various thicknesses for each 
substance and 2 calculated; the value given in the first column of the table is 
practically a mean of such determinations, special importance being attached to 
what were considered the more accurate observations. The second column shows 
the value of \ as determined by the second method ; the agreement is good except 
in the case of copper, and the explanation of the difference in this case has 
not been noticed. In the succeeding columns the values of \ given in the first 
column have been used :— 
r | r vA | A | Me Atomic 
SUBSTANCE. | —. [b= —— —. a 
¢ sae First Method. |Second Method.| Density /1=« | Density Weight. 
a ie a , : 
Lead, . | 91 95 8-0 2€8 93-4 | 206-4 
Platinum, 173 Wm | 8O | ayo 22:3 | 194-3 
Ma, vai | 54 50 7:4 133 18-2 118°8 
Cadmium, . 66 65 75 160 18°6 112-0 
Silver, . : 85 83 SO | 2038 We | NOS 
Zinc, . ‘ 48 54 66 103 14:3 65:4 
Coppers |e: 50 61 5:6 107 120 | 63-2 
Nickel, . : 63 63 7:6 127 15°3 59:0 
——$———— | ee med — ee ee ee 
Aluminium, . 14 — 5:0 23 SH | Zi On| 
I | 
Discussion oF RESUL'S. 
The ratio of the coefficient \ to the density varies very considerably, and, 
what is more important, the variation is irregular. We learn little, therefore, as to 
the stopping power of different types of atoms by measuring this coefficient \ for 
equal weight thicknesses of different forms of matter. When we turn to the 
coefficient , the manner of its variation is at once apparent. It will be remem- 
bered that » is what the coefficient of absorption would be if there were no 
secondary 6 rays; it is the érwe coefficient of absorption of any given set of 
particles. As shown above, 
where « is the ratio of the energy of the secondary particles emitted from an 
element of volume to that of the particles absorbed in that element. The values 
of « are taken from the previous paper. ‘The column in the table giving the 
TRANS. ROY. DUB. SOC., N.S,, VOL. IX., PARI Ly. K 
