A simple method of determining the absorption in gases is shown in Fig. 36. The maximum current is measured between two parallel plates _A_ and _B_ kept at a _fixed_ distance of 2 cms. apart, and then moved by means of a screw to different distances from the radio-active surface. The radiation from this active surface passed through a circular opening in the plate _A_, covered with thin aluminium foil, and was stopped by the upper plate. For observations on other gases besides air, and for examining the effect at different pressures, the apparatus is enclosed in an air-tight cylinder. If the radius of the active surface is large compared with the distance of the plate _A_ from it, the intensity of the radiation is approximately uniform over the opening in the plate _A_, and falls off with the distance _x_ traversed according to an exponential law. Thus $$ \frac {I} {I₀} = e^{–λ x} $$, where λ is the “absorption constant” of the radiation for the gas under consideration[163]. Let _x_ = distance of lower plate from active material, _l_ = distance between the two fixed plates. The energy of the radiation at the lower plate is then $$ I₀ e^{–λ x} $$, and at the upper plate $$ I₀ e^{–λ (l + x)} $$ . The total number of ions produced between the parallel plates _A_ and _B_ is therefore proportional to $$ e^{–λ x} − e^{–λ (l + x)} = e^{–λ x} (1 - e^{–λ l}) $$ . Since the factor $$ 1 = e^{–λ l} $$ is a constant, the saturation current between _A_ and _B_ varies as $$ e^{–λ x} $$, _i.e._ it decreases according to an exponential law with the distance traversed. [Illustration: Fig. 37.] The variation of the current between _A_ and _B_ with the distance from a thin layer of uranium oxide is shown in Fig. 37 for different gases. The initial measurements were taken at a distance of about 3·5 mms. from the active surface. The actual values of this initial current were different for the different gases, but, for the purposes of comparison, the value is in each case taken as unity. It will be seen that the current falls off with the distance approximately in a geometrical progression, a result which is in agreement with the simple theory given above. The distance through which the rays pass before they are absorbed is given below for different gases. Gas Distance in mms. to absorb half of radiation Carbonic acid 3 Air 4·3 Coal-gas 7·5 Hydrogen 16 The results for hydrogen are only approximate, as the absorption is small over the distance examined.