. Radio-activity . ifferent for the different gases, but, for the purposesof comparison, the value is in each case taken as unity. It will be seen that the current falls off with the distanceapproximately in a geometrical progression, a result which is inagreement with the simple theory given above. The distancethrough which the rays pass before they are absorbed is givenbelow for different gases. Gas Carbonic acidAir Coal-gasHydrogen ... Distance in mms. toabsorb half of radiation 343 7-516 The results for hydrogen are only approximate, as the absorp-tion is small over the distance examined.
. Radio-activity . ifferent for the different gases, but, for the purposesof comparison, the value is in each case taken as unity. It will be seen that the current falls off with the distanceapproximately in a geometrical progression, a result which is inagreement with the simple theory given above. The distancethrough which the rays pass before they are absorbed is givenbelow for different gases. Gas Carbonic acidAir Coal-gasHydrogen ... Distance in mms. toabsorb half of radiation 343 7-516 The results for hydrogen are only approximate, as the absorp-tion is small over the distance examined. 136 NATURE OF THE RADIATIONS [CH. The absorption is least in hydrogen and greatest in carbonicacid, and follows the same order as the densities of the the case of air and carbonic acid, the absorption is proportionalto the density, but this rule is widely departed from in the caseof hydrogen. Results for the relative absorption by air of the a. raysfrom the different active bodies are shown in Fig. 30. 100. 10 12 14 16Ditsta?icc in mms. 26 Fig. 30. The initial observation was made about 2 mms. from the activesurface, and the initial current is in each case taken as 100. Thecurrent, as in the case of uranium, falls off at first approximatelyin geometrical progression with the distance. The thickness ofair, through which the radiation passes before the intensity isreduced to half value, is given below. Distance in mms. Uranium 4*3 Radium 75 Thorium 10 Excited radiation from Thorium and Radium ... 16-5 The order of absorption by air of the radiations from the activesubstances is the same as the order of absorption by the metalsand solid substances examined. IV] NATURE OF THE RADIATIONS 137 93. Connection between absorption and density. Sincein all cases the radiations first diminish approximately accordingto an exponential law with the distance traversed, the intensity /after passing through a thickness x is given by / = I0e~kx where Xis the absorption constant and I0 the
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