where _i_ is the current for a thickness _d_, and _i₀_ the initial current. In the case of polonium, the decrease is far more rapid than would be indicated by the exponential law. By the first layer, the current is reduced to the ratio ·41. The addition of the third layer cuts the current down to a ratio of ·17. For most of the active bodies, the current diminishes slightly faster than the exponential law would lead one to expect, especially when the radiation is nearly all absorbed. =98.= The increase of absorption of the α rays of polonium with the thickness of matter traversed has been very clearly shown in some experiments made by Mme Curie. The apparatus employed is shown in Fig. 34. [Illustration: Fig. 34.] The saturation current was measured between two parallel plates _PP´_ 3 cms. apart. The polonium _A_ was placed in the metal box _CC_, and the rays from it, after passing through an opening in the lower plate _P´_, covered with a layer of thin foil _T_, ionized the gas between the plates. For a certain distance _AT_, of 4 cms. or more, no appreciable current was observed between _P_ and _P´_. As the distance _AT_ was diminished, the current increased in a very sudden manner, so that for a small variation of the distance _AT_ there was a large increase of current. With still further decrease of distance the current increases in a more regular manner. The results are shown in the following table, where the screen _T_ consisted of one and two layers of aluminium foil respectively. The current due to the rays, without the aluminium screen, is in each case taken as 100. Distance AT in cms. 3·5 2·5 1·9 1·45 0·5 For 100 rays 0 0 5 10 25 transmitted by one layer For 100 rays 0 0 0 0 0·7 transmitted by two layers The metallic screen thus cuts off a greater proportion of the rays the greater the distance of air which the radiations traverse. The effects are still more marked if the plates _PP´_ are close together. Results similar but not so marked are found if radium is substituted for the polonium. It follows from these experiments that the ionization per unit volume, due to a large plate uniformly covered with the radio-active matter, falls off rapidly with the distance from the plate. At a distance of 10 cms. the α rays from uranium, thorium, or radium have been completely absorbed in the gas, and the small ionization then observed in the gas is due to the more penetrating β and γ rays. The relative amount of the ionization observed at a distance from the source will increase with the thickness of the layer of active matter, but will reach a maximum for a layer of a certain thickness. The greater proportion of the ionization, due to unscreened active matter, is thus entirely confined to a shell of air surrounding it not more than 10 cms. in depth. [Illustration: Fig. 35.]