Page:Radio-activity.djvu/202

 Table B gives the values of λ divided by density for the above numbers. If the absorption were directly proportional to the density, the quotient would be the same in all cases.

TABLE B.

λ divided by density.

++++-++ ++++-++ ++++-++
 * Substance |   I    |   II   |   III   |   IV   |
 * Platinum    | ·054   |        |         |        |
 * Mercury     | ·053   | ·048   |  ·039   |  ·036  |
 * Lead        | ·056   | ·049   |  ·042   |  ·037  |
 * Zinc        | ·039   | ·037   |  ·034   |  ·033  |
 * Aluminium   | ·038   | ·038   |  ·038   |  ·038  |
 * Glass       | ·034   | ·034   |  ·034   |  ·034  |
 * Water       | ·034   | ·034   |  ·034   |  ·034  |

The numbers in column I vary considerably, but the agreement becomes closer in the succeeding columns, until in column IV the absorption is very nearly proportional to the density.

It is seen that the absorption of all three types of rays from radio-active substances is approximately proportional to the density of the substance traversed—a relation first observed by Lenard for the cathode rays. This law of absorption thus holds for both positively and negatively electrified particles projected from the radio-active substances, and also for the electromagnetic pulses which are believed to constitute the γ rays; although the absorption of the [Greek: alpha] rays, for example, is 10,000 times greater than for the γ rays. We have seen in section 84 that the value of the absorption constant λ for lead is 122 for the β rays from uranium. The value for the γ rays from radium varies betwen ·64 and ·44, showing that the γ rays are more than 200 times as penetrating as the β rays.

107. Nature of the rays. In addition to their great penetrating power, the γ rays differ from the [Greek: alpha] and β rays in not being deflected to an appreciable degree by a magnetic or electric field. In a strong magnetic field, it can be shown, using the photographic method, that there is an abrupt discontinuity between the β and γ rays, for the former are bent completely away