Electric Field of the «, /3, and Secondary Rays. 721 
supposition values of the coefficients of absorption by alu- 
minium were given for various radiators. It has since been 
shown bv S. J. Allen*, and others, that the secondary elec- 
trons really move with velocities as widely different as those 
of the j3 rays themselves. The mistake in my paper arose 
partly from working with too thick an aluminium face 
to the electroscope, and from using too light a substance, 
namely paper, as radiator. Allen also proved that the 
secondary radiations from lead or zinc had penetrating powers 
and, therefore, velocities almost as great as those of the 
particles from radium C ; but that is not the case for light 
substances such as wood, paper, aluminium or paraffin. 
It seems desirable in the first place to give corrected values f 
for the coefficients of absorption by aluminium in the case of 
some different secondary radiating substances, because these 
results have considerable bearing on the subsequent and main 
work described in this communication. 
The values of \ for secondary rays have been determined 
in the following manner : — 
About fourteen milligrams of pure radium bromide were 
placed in two sealed glass test-tubes. The primary /3 and 
7 rays struck the radiating plates, which were several centi- 
metres thick, and had an area of 22 x 22 square centimetres. 
The secondary incident rays were tested in a gold-leaf 
electroscope of small natural leak, measuring 10 X 10 x 16 c.c. 
One of the larger faces was removed, a few fine wires were 
stretched across, and the open side was than covered with thin 
aluminium leaf 0*00031 cm. thick. Rutherford has found that 
passage through such a sheet of aluminium reduces the length 
of the range in air of the a particle by 0*5 cm. only. It is 
certain, therefore, that such a layer of aluminium will 
scarcely affect the passage of # or of secondary rays. In 
front of the electroscope aluminium screens could be placed ; 
both these and the electroscope were separated from the radium 
by 15 cms. thickness of lead. The coefficients of absorption 
were calculated in the ordinary way between the various thick- 
nesses stated in the first column of Table I. In the first 
and second rows of figures the atomic weights and secondary 
radiation values are stated. In the second column are the 
values of A, for primary rays, and in the other columns the 
values for secondary rays from the various substances named. 
* Phys. Review, August 1903. 
t Since this paper was forwarded for publication I have read a com- 
munication by H. Starke in Le Radium (Feb. 1908). He had obtained 
results with which those given in the first part of this paper are in general 
agreement. 
