Platinum 1·167 Mercury ·726 ·661 ·538 ·493 Lead ·641 ·563 ·480 ·440 Zinc ·282 ·266 ·248 ·266 Aluminium ·104 ·104 ·104 ·104 Glass ·087 ·087 ·087 ·087 Water ·034 ·034 ·034 ·034 In the above table, the absorption in aluminium, glass and water was too small to determine with accuracy the variation of λ with distance traversed. It will be observed that, for the denser substances, the coefficient of absorption decreases with the distance through which the rays have passed. This indicates that the rays are heterogeneous. The variation of λ is more marked in heavy substances. 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 α 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 between ·64 and ·44, showing that the γ rays are more than 200 times as penetrating as the β rays.