An interesting result follows from the rapid absorption of radiation by the gas. If the current is observed between two fixed parallel plates, distant _d₁_ and _d₂_ respectively from a large plane surface of active matter, the current at first increases with diminution of pressure, passes through a maximum value, and then diminishes. In such an experimental case the lower plate through which the radiations pass is made either of open gauze or of thin metal foil to allow the radiation to pass through readily. The saturation current _i_ is obviously proportional to $$ \int_{d_1}^{d_2} pI₀e^{-pλ_1 d} $$, i.e. to $$ \frac{I₀}{λ_1} (e^{-pλ_1 d_1} − e^{-pλ_1 d_2}) $$ This is a function of the pressure, and is a maximum when $$ \log_e \frac{d_1}{d_2} = − pλ_1 (d_2 − d_1) $$ For example, if the active matter is uranium, _p_λ₁ = 1·6 for the α rays at atmospheric pressure. If _d₂_ = 3, and _d₁_ = 1, the saturation current reaches a maximum when the pressure is reduced to about ⅓ of an atmosphere. This result has been verified experimentally. =45. Conductivity of different gases when acted on by the rays.= For a given intensity of radiation, the rate of production of ions in a gas varies for different gases and increases with the density of the gas. Strutt[76] has made a very complete examination of the relative conductivity of gases exposed to the different types of rays emitted by active substances. To avoid correction for any difference of absorption of the radiation in the various gases, the pressure of the gas was always reduced until the ionization was directly proportional to the pressure, when, as we have seen above, the ionization must everywhere be uniform throughout the gas. For each type of rays, the ionization of air is taken as unity. The currents through the gases were determined at different pressures, and were reduced to a common pressure by assuming that the ionization was proportional to the pressure. With unscreened active material, the ionization is almost entirely due to α rays. When the active substance is covered with a layer of aluminium ·01 cm. in thickness, the ionization is mainly due to the β or cathodic rays, and when covered with 1 cm. of lead, the ionization is solely due to the γ or very penetrating rays. Experiments on the γ rays of radium were made by observing the rate of discharge of a special gold-leaf electroscope filled with the gas under examination and exposed to the action of the rays. The following table gives the relative conductivities of gases exposed to various kinds of ionizing radiations. Gas Relative α β γ Röntgen Density rays rays rays rays Hydrogen 0·0693 0·226 0·157 0·169 0·114 Air 1·00 1·00 1·00 1·00 1·00 Oxygen 1·11 1·16 1·21 1·17 1·39 Carbon dioxide 1·53 1·54 1·57 1·53 1·60