Taking into account the difficulties of the manipulation of very thin screens and of the superposition of screens in contact, the numbers of each column may be looked upon as constant; the first number only of the aluminium column indicates a greater absorption than that indicated by the following numbers. The α-rays of radium behave similarly to the rays of polonium. These rays may be investigated almost isolated by deflecting to one side the β-rays with the magnetic field; the γ-rays seem of slight importance in comparison with the α-rays. The operation can only be carried on at some distance from the source of radiation. The following are the results of an experiment of this kind. The fraction of the radiation transmitted by a lamina of aluminium 0·01 m.m. thick is measured; this screen was placed always in the same position, above and at a little distance from the source of radiation. With the apparatus of Fig. 5, the current produced in the condenser for different values of the distance A D is observed, both with and without the screen:— Distance A D 6·0 5·1 3·4 Percentage of rays transmitted by the aluminium 3 7 24 The rays which travel furthest in the air are those most absorbed by the aluminium. There is therefore a great similarity between the absorbable α-rays of radium and the rays of polonium. The deflected β-rays and the undeflected penetrating γ-rays are, on the contrary, of a different nature. The experiments, notably of MM. Meyer and von Schweidler, clearly show that, considering the radiation of radium as a whole, the penetrating power of this radiation increases with the thickness of the material traversed, as is the case of Röntgen rays. In these experiments the α-rays produce scarcely any effect, being for the most part suppressed by very thin absorbent screens. Those which penetrate are, on the one hand, β-rays more or less scattered; on the other hand, γ-rays, which appear similar to Röntgen rays. The following are the results of some of my experiments on the subject:— The radium is enclosed in a glass vessel. The rays, which emerge from the vessel, traverse 30 c.m. of air, and are received upon a series of glass plates, each of thickness 1·3 m.m.; the first plate transmits 49 per cent of the radiation it receives, the second transmits 84 per cent of the radiation it receives, the third transmits 85 per cent of the radiation it receives. In another series of experiments the radium was enclosed in a glass vessel placed 10 c.m. from the condenser which received the rays. A series of similar screens of lead each 0·115 m.m. thick were placed on the vessel. The ratio of the radiation transmitted to the radiation received is given for each of the successive screens by the following numbers:— 0·40 0·60 0·72 0·79 0·89 0·92 0·94 0·94 0·97