
378 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
the other hand, the results are to be expected if the ionization 
of the enclosed air is mainly due to an easily absorbed radiation 
from the walls of the vessel. If this radiation had a penetrating 
power about equal to that observed for the a rays of the radio- 
elements, the radiation would be absorbed in a few centimetres of 
air. With diminution of pressure, the radiations would traverse 
a greater distance of air before complete absorption, but the total 
lonization produced by the rays would still remain about the same, 
until the pressure was reduced sufficiently to allow the radiation 
to traverse the air space in the vessel without complete absorption. 
With still further diminution of pressure, the total ionization 
produced by the radiation, and in consequence the current observed, 
will vary directly as the pressure. 
220. Examination of ordinary matter for radio-activity. 
Strutt?, McLennan and Burton’, and Cooke’, independently ob- 
served about the same time that ordinary matter is radio-active 
to a slight degree. Strutt, by means of an electroscope, observed 
that the ionization produced in a closed vessel varied with the 
material of the vessel. A glass vessel with a removeable base 
was employed and the vessel was lined with the material to be 
examined. The following table shows the relative results obtained. 
The amount of leakage observed is expressed in terms of the 
number of scale divisions of the eye-piece passed over per hour 
by the gold-leaf: 



| . 
Material of lining of vessel | eee meee 
cana | 
| Tinfoil... a ef | 3°3 
| » another sample x eet 2°3 
Glass coated with phosphoric acid 1°3 
| Silver chemically deposited on glass 1°6 
Zinc 1°2 
Lead ae 2°2 
Copper (clean) 2°3 
5 (oxidized) sols Le 7/ 
| Platinum (various samples) i} 2°0, 2°9, 3°9 
| Aluminium 1°4 
1 Phil. Mag. June, 1903. Nature, Feb. 19, 1903. 
* Phys. Rev. No. 4, 1903. J.J. Thomson, Nature, Feb. 26, 1903. 
3 Phil. Mag. Aug. 6, 1903. Rutherford, Nature, April 2, 1903. 
