148 Mr. 8. J. Allan on the 
a given thickness of gas. A fixed distance, about 6 mms., 
between the leather and the aluminium, was always taken as 
a basis from which to calculate the percentage of unabsorbed 
rays. 
Tf the radius of the active surface is large compared with 
its distance from the aluminium foil, it can be readily shown 
from the ionization theory that the following equation holds : 
I = jena 
I, is the intensity of the radiation after passing through a 
distance y of the gas, and X the coefficient of absorption of 
the gas considered. ‘The percentage of the radiation unab- 
sorbed is calculated in the same way as for solids. The 
results of the experiments are shown plotted in fig. 9 
(Pl. XIV.), the ordinates giving the percentage of rays 
unabsorbed after passing through a certain distance, and the 
abscissee the turns of the screw-head, each turn corresponding 
to 1:27 mm. These results are compared with those of 
aluminium in the following table. 











Tapa DV. 
Radiation | / | 
Substance. reduced to half r. Density. —,. | 
its value in Density | 
ALMIMINTUM. . 255055. cece 00076 cm. 1050 2°6 | 400 
Ripe h 28 a ioe ae 54) 0012 450 | 
Carbonic Acid Gas ...| °76 i ‘91; -0018 500 | 
Coall' Gas) 02) )s.004009% ievaleeee, ‘41; -00048 840 | 
Pivcocen. alee 5:56 19, -000083 | 2300 | 

It will be seen that the absorption by gases follows the 
order of their densities, and is almost proportional to density 
for air and carbonic-acid gas. 
Increased Conductivity of Air mixed with Water Spray. 
J.J. Thomson * describes some experiments in which the 
conductivity of air was increased by passing it through a 
water-pump into a large vessel, where it was tested. He also 
found that when a brass rod was suspended in this vessel and 
kept charged for a number of hours to a high negative 
potential, it had acquired a certain amount of excited 
activity f. 
* Phil. Mag. Sept. 1902. 
+ Note.—The effects observed by J. J. Thomson have since been shown 
by him to be due to a radioactive emanation present in the tap-water of 
Cambridge. 
