Page:Proceedings of the Royal Society of London Vol 60.djvu/161

146 If the Lorenz expression for S, namely, -. be preferred to d f.i—1/d, it may be substituted in either of the above formulae.

In either case the actual numbers will, of course, be changed more or less, but the relation above pointed out will still hold good. The discrepancies will, however, be somewhat exaggerated by the change.

This is suggested as a first approximation to a new law. It may be useful in both chemical and physical science. It holds good, however, only for the metallic elements.

The experiments described in this paper were made to determine whether or not the Rontgen rays given off by a vacuum bulb were of a homogeneous nature, by examining the manner in which they are absorbed by different substances. The induction coil and vacuum bulb for producing the rays were enclosed in a wooden box thickly lined with metal, with a small hole in the top, directly beneath which and close up to it the vacuum bulb was placed. Over the hole a well-insulated metal disk was placed and connected to one pair of quadrants of an electrometer. The two pairs of quadrants are first connected together and with one terminal of a battery of small storage cells, the other terminal being connected to earth.

The quadrants of the electrometer are then separated from each other and from the storage cells, and the induction coil turned on. The Rontgen rays passing through the hole in the box and falling on the charged disk discharges it, and the intensity of the radiation is measured by the rate at which the spot of light from the electrometer needle moves across the scale. The metal lining of the box is connected to earth, and the small hole covered with a single sheet of tinfoil to screen the electrometer from direct electrical disturbances.

The substance whose absorptive power is to be examined is placed over the hole so that the rays traverse it before falling on the charged disk.

Evidences of selective absorption were sought for in the following manner. The rate of leakage was accurately determined when the rays were passing through one of the substances used, say a plate of glass. Sheets of tinfoil were then substituted for the glass and the