number_n, say—taken such that the leakage from the charged disk was approximately the same as when the glass was used. The rate of leak was then measured accurately. The ratio of the rate of leak with the glass to that with the n sheets of tinfoil gives a measure of their relative transparency to Rontgen rays.
A number of tinfoil sheets is now placed over the hole; the glass plate is placed on the top, and the rate of leak measured. The glass is removed and the same n sheets of tinfoil as were formerly used put in its place, and the leakage measured. The ratio of the rate of leak in the latter two cases is a measure of the relative transparency of the glass and the n tinfoil sheets to the Rontgen rays after they have been already screened by passing through several layers of tinfoil.
The two ratios thus obtained should be equal if the Rontgen rays are all of one kind, but if the glass is relatively less transparent in the second case it can only be explained by assuming that the Rontgen rays are not homogeneous, and that some of them are more readily absorbed by the glass and others by the tinfoil.
Various substances were tested against tinfoil in the manner described. With some there was no selective absorption, with others it was very marked. Glass gave none, with mica and paraffin the effect was small, with fuchsine, eosine, fluorescine, aesculin, and barium sulphide the effect was very marked. With several fluorescent screens the effect was also marked. Pure water also gave a distinct though smaller effect.
The table below sets forth the results obtained with these substances.
Column B gives the quotient of the rate of leak through the substance in column A to that through a number of tinfoil layers which gave approximately the same leak. Column C gives the quotient of the rate of leak through the substance to that through the same tinfoil layers after the rays have already passed through four layers of tinfoil.
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A. B. C. Difference. Calcium tungstate............ , 1 07 0*85 0*22 Calcium platinocyanide .. .. ISO 0*86 0-44 Luminous paint................... 1-0 0*71 0*29 Potassium platinoeyanide .. 1 -10 0*87 0*23 Fuchsine............................... 1M5 0*77 0*38 Eosine........................... , .. . 1 ’31 1*00 0*31 Aesculin............................... 1*33 0*90 0*43 Fluorescine. .......................... 1*32 1*08 0 *24 Barium sulphide................. 1*30 0*97 0*33
