42 
PKOFESSOK E. EUTHERFOEI) AXD mi E. K. McCErXC4 
Pressure in 
atmospheres. 
A alue of A. 
• 5 
1 
•000158 
1 
•00028S 
0 
•00060 
o 
O 
•00086 
The value of X at atmospheric pressure and temperature obtained for the same bulb 
after daily use for tv'o months ^vas found to be '000270. The value of ‘X at 
atmosjjheric pressure and temperature in the calculations is taken as the mean ot 
these two values, and is thus oiven l^v 
X = -000279. 
It is probable that the rays were not homogeneous, and the value of X must be 
considered as the mean value for the different kinds of ravs. On account of the verv 
t/ 
small absorption of the rays it was difficult to deteiinine with accuracy the absorption 
for pressures lower than half an atmosphere. Tlie results, however, indicated that 
the absorpticm was, roughly, proportional to the pressure for still lower pressures. 
The value of X was determined for carbonic acid gas at normal pi'essure and 
temperature. The absorption was 1'59 times that of air, and the value of X was 
found to be '000457. The results wei'e contirmed hy varying the pressure of the air 
in one tid^e until there was no disturbance of the electrometer zero. The results 
agreed with the value obtained above, assuming the absorption in air is pro23ortional 
to the pressure. 
We see from the results given above that the radiation is reduced to half its value 
with no absorption after passing through a length of 24'7 metres of air at ordinary 
pressure and tem})erature. 
The value of X for uranium rays^ is 1'6, or the absorption is 6000 times as great 
for uranium rays as for the X rays employed. The value of X obtained some years 
ago for a much “ softer ” l)idl) was '001, or about four times the absorption of the bulb 
employed in these ex})eriments. 
Mcasurcrncnt of the Current through the Gas. 
In order to determine the amount of ionization in a known volume ol gas, the 
apparatus shown in fig. 6 was employed. 
The rays jtassed into a l)rass cylinder, 12 centims. in diameter and 30 centims. in 
length, through a rectangular orifice, O, at one end covered with an aluminium 
window, 1 mihim. in thickness. Inside the cylinder two parallel I'ectangidar plates, 
* E. ErTiiEKFORr), ‘Phil. Mag.,’ January, 1899. 
