[EVE] ABSORPTION BY AIR OF THE BETA RAYS 61 
The f rays were then cut off to a great extent by two sheets of aluminium, 
each 1-7 mm. thick, placed at the radium. These would permit about 
one per cent of 8 rays to pass through them, and would reduce the y 
radiation by about two per cent. 
After deducting the natural leak, the readings in divisions per 
minute were taken and the results obtained are as follows:—where I is 
the ionization due to # rays, and I’ to y rays. 
TABLE I. 
Metres | I + I’ IY ere I log Tr? u 
1 69 12-5 12-5 56-5 4-03 
OREO Ra Os ETE -0029 
2 14-3 3°78 15-1 10-52 | 3-74 
SR Se que Re AO 2 PVR -0048 
3 4-47 1-57 14.1 2.90 | 3-26 
MR CANT ane Beha ee -0042 
4 1-94 +87 13-9 1-07 | 2-84 
RÉ NES Cree es eR -0041 
5 1-02 -57 14-1 -45 | 2-43 
RE TAN ees TU -0051 
6 -58 +39 14-0 -19 | 1-92 
u = -0044 cm. 
These results are also shown in Figure 1, where the abscissae are 
given in metres and the ordinates are the logarithms of I in divisions 
for one minute, multiplied by the distance squared in metres. The 
upper curve represents the ff ray measurements. At one metre the 
ordinate appears to be too small, possibly because of insufficient voltage 
to secure saturation or because of some other effect requiring a special 
investigation. The average slope for distances most favorable to ac- 
. al 
curate measurements gives {{—:*0044 cm. 
The lower curve is for y rays, and a straight line is anticipated. 
The initial rise is not easy to explain; the point at 2 metres is too high 
owing to accidental conditions. In this experiment it is certain that 
some f particles passed through the aluminium screens, and that others 
were produced by the y rays at emergence from the screens near the 
radium. 
Therefore, some experiments were made in which the test tube 
containing the radium were placed inside a cylinder of zine between 
the poles of a powerful electromagnet, and the magnetic field was in- 
