Page:Radio-activity.djvu/559

 *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 removable 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   | Leakage in scale |
 * |divisions per hour|
 * Tinfoil                            |       3·3        |
 * "   another sample              |       2·3        |
 * Glass coated with phosphoric acid  |       1·3        |
 * Silver chemically deposited on glass|      1·6        |
 * Zinc                               |       1·2        |
 * Lead                               |       2·2        |
 * Copper (clean)                     |       2·3        |
 * "   (oxidized)                   |       1·7        |
 * Platinum (various samples)         |  2·0, 2·9, 3·9   |
 * Aluminium                          |       1·4        |

There are thus marked differences in the leakage observed for different materials and also considerable differences in different samples of the same metal. For example, one specimen of platinum caused nearly twice the leakage of another sample from a different stock.

McLennan and Burton, on the other hand, measured by means of a sensitive electrometer the ionization current produced in the air in a closed iron cylinder 25 cms. in diameter and 130 cms. in length, in which an insulated central electrode was placed. The open cylinder was first exposed for some time at the open window of the laboratory. It was then removed, the top and bottom closed, and the saturation current through the gas determined as soon as possible. In all cases it was observed that the current diminished for two or three hours to a minimum and then very slowly increased again. In one experiment, for example, the initial current observed corresponded to 30 on an arbitrary scale. In the course of four hours the current fell to a minimum of 6·6, and