The thickness required to stop a given proportion of the β rays thus decreases with the density, but not nearly so fast as the density increases. These results are difficult to reconcile with the density-law of absorption found by Lenard from the cathode rays, or with the results of the ionization method already considered. A further experimental examination of the whole question is very much to be desired. =86. Variation of the amount of radiation with the thickness of the layer of radiating material.= The radiations are sent out equally from all portions of the active mass, but the ionization of the gas which is measured is due only to the radiations which escape into the air. The depth from which the radiations can reach the surface depends on the absorption of the radiation by the active matter itself. Let λ be the absorption constant of the homogeneous radiation by the active material. It can readily be shown that the intensity _I_ of the rays issuing from a layer of active matter, of thickness _d_, is given by $$ \frac {I} {I_{0}} = 1 − e^{–λ d} $$, where _I₀_ is the intensity at the surface due to a very thick layer. This equation has been confirmed experimentally by observing the current due to the β rays for different thicknesses of uranium oxide. In this case _I_ = (½)_I₀_ for a thickness of oxide corresponding to ·11 gr. per sq. cm. This gives a value of λ divided by density of 6·3. This is a value slightly greater than that observed for the absorption of the same rays in aluminium. Such a result shows clearly that the substance which gives rise to the β rays does not absorb them to a much greater extent than does ordinary matter of the same density. The value of λ will vary, not only for the different active substances, but also for the different compounds of the same substance. PART III. The α Rays. =87. The α rays=. The magnetic deviation of the β rays was discovered towards the end of 1899, at a comparatively early stage in the history of radio-activity, but three years elapsed before the true character of the α rays was disclosed. It was natural that great prominence should have been given in the early stages of the subject to the β rays, on account of their great penetrating power and marked action in causing phosphorescence in many substances. The α rays were, in comparison, very little studied, and their importance was not generally recognized. It will, however, be shown that the α rays play a far more important part in radio-active processes than the β rays, and that the greater portion of the energy emitted in the form of ionizing radiations is due to them.