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 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:—

The two plates of a condenser, and  (Fig 8), are horizontally disposed in a metallic box,, connected to earth. The active body,, placed in a thick metallic box,, connected with the plate , acts upon the air of the condenser across a metallic sheet, ; the rays which pass through the sheet are alone utilised for producing the current, the electric field being limited by the sheet. The distance,, of the active body from the sheet may be varied. The field between the plates is established by means of a battery. By placing in upon the active body different screens, and by adjusting the distance, the absorption of rays which travel long or short distances in the air may be determined.

The following are the results obtained with polonium:—

For a certain value of the distance (4 c.m. and more), no current passes; the rays do not penetrate the condenser. When the distance is diminished, the appearance of the