Page:Radio-activity.djvu/157

 It will be observed that the value of the absorption constant divided by the density is very nearly the same for such different

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 * Substance | [Greek: lambda] | Density | [Greek: lambda] |
 * |         |         | Density  |/Density]
 * Glass    |   14·0   |   2·45  |    5·7   |
 * Mica     |   14·2   |   2·78  |    5·1   |
 * Ebonite  |    6·5   |   1·14  |    5·7   |
 * Wood     |    2·16  |    ·40  |    5·4   |
 * Cardboard |   3·7   |    ·70  |    5·3   |
 * Iron     |   44     |   7·8   |    5·6   |
 * Aluminium |  14·0   |   2·60  |    5·4   |
 * Copper   |   60     |   8·6   |    7·0   |
 * Silver   |   75     |  10·5   |    7·1   |
 * Lead     |  122     |  11·5   |   10·8   |
 * Tin      |   96     |   7·3   |   13·2   |

substances as glass, mica, ebonite, wood, iron and aluminium. The divergences from the law are great, however, for the other metals examined, viz. copper, silver, lead and tin. In tin the value of [Greek: lambda] divided by the density is 2·5 times its value for iron and aluminium. These differences show that a law for the absorption of the [Greek: beta] rays depending only on the density does not hold for all substances. With an exception in the case of tin, the value of [Greek: lambda] divided by the density for the metals increases in the same order as their atomic weights.

The absorption of the [Greek: beta] rays by matter decreases very rapidly with increase of speed. For example, the absorption of cathode rays in Lenard's experiment (loc. cit.) is about 500 times as great as for the uranium [Greek: beta] rays. The velocity of the [Greek: beta] rays of uranium was found by Becquerel to be about 1·6 × 10^{10} cms. per sec. The velocity of the cathode rays used in Lenard's experiment was certainly not less than 1/10 of this, so that, for a decrease of speed of less than 10 times, the absorption has increased over 500 times.

85. Number of electrons stopped by matter. An account will now be given of the experiments made by Seitz, to determine