The two plates of a condenser, P P and P′ P′ (Fig 8), are horizontally disposed in a metallic box, B B B B, connected to earth. The active body, A, placed in a thick metallic box, C C C C, connected with the plate P′ P′, acts upon the air of the condenser across a metallic sheet, T; the rays which pass through the sheet are alone utilised for producing the current, the electric field being limited by the sheet. The distance, A T, 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 A upon the active body different screens, and by adjusting the distance A T, the absorption of rays which travel long or short distances in the air may be determined. [Illustration: FIG. 8.] The following are the results obtained with polonium:— For a certain value of the distance A T (4 c.m. and more), no current passes; the rays do not penetrate the condenser. When the distance A T is diminished, the appearance of the rays in the condenser is manifested somewhat suddenly, a weak current changing to one of considerable strength for a slight diminution of distance; the current then increases regularly as the active body continues to approach the sheet T. When the active body is covered with a sheet of aluminium 1/100 m.m. thick, the absorption produced by the lamina becomes greater, the greater the distance A T. If a second similar lamina of aluminium be placed upon the first, each absorbs a fraction of the radiation it receives, and this fraction is greater for the second lamina than for the first. In the following table I have represented in the first line the distances in centimetres between the polonium and the sheet T; in the second line the percentage of the rays transmitted by a sheet of aluminium; in the third line the percentage of the rays transmitted by two sheets of the same aluminium:— Distance A T 3·5 2·5 1·9 1·45 0·5 Percentage of rays transmitted by one lamina 0 0 5 10 25 Percentage of rays transmitted by two laminæ 0 0 0 0 0·7 In these experiments the distance of the plates, P and P′, was 3 c.m. We see that the interposition of the aluminium screen diminishes the intensity of the radiation to a greater degree at further distances than at nearer distances. This effect is still more marked than the preceding figures seem to indicate. For a distance of 0·5 c.m. 25 per cent represents the mean penetration for all the rays which pass beyond this distance. If, for example, only those rays between 0·5 c.m. and 1 c.m. be comprehended, the penetration would be greater. And if the plate P be placed at a distance of 0·5 c.m. from P′ the fraction of the radiation transmitted by the aluminium lamina (for A T = 0·5 c.m.) is 47 per cent, and through two laminæ it is 5 per cent of the original radiation.