_a._ Fraction of radiation transmitted. Chloride of barium and radium 57 0·32 Bromide of barium and radium 43 0·30 Chloride of barium and radium 1200 0·30 Sulphate of barium and radium 5000 0·29 Sulphate of barium and radium 10,000 0·32 Metallic bismuth and polonium 0·22 Compounds of uranium 0·20 Compounds of thorium in a thin layer 0·38 We see that radium compounds of different nature and activity give very similar results, as I have already pointed out in the case of uranium and thorium compounds at the beginning of this work. We see also that, taking into account the whole of the radiation, and with a given absorbent screen, the different radio-active bodies can be arranged in the following decreasing order of penetrating power:—Thorium, radium, polonium, uranium. These results are similar to those which have been published by Mr. Rutherford. Mr. Rutherford also finds that the order is the same when air is the absorbent substance. But it is probable that this order has no absolute value, and would not be maintained independently of the nature and thickness of the screen. Experiment shows, indeed, that the law of absorption is very different for polonium and radium, and that, for the latter, the absorption of the rays of each of the three groups must be considered separately. Polonium is particularly well adapted to the study of α-rays, because the specimens which we possess emit no other kind of rays. I made a preliminary series of experiments with extremely active recently prepared specimens of polonium. I found the absorbability of the rays to increase with increase of thickness of the matter traversed. This singular law of absorption is contrary to that known for other kinds of radiation. I employed for this research our apparatus for the determination of electrical conductivity arranged in the following manner:—