The rate at which the emanation is produced is independent of the gas surrounding the active matter. If in the apparatus of Fig. 51 air is replaced by hydrogen, oxygen, or carbonic acid, similar results are obtained, though the current observed in the testing vessel varies for the different gases on account of the unequal absorption by them of the radiation from the emanation. If a thorium compound, enclosed in paper to absorb the α radiation, is placed in a closed vessel, the saturation current due to the emanation is found to vary directly as the pressure. Since the rate of ionization is proportional to the pressure for a constant source of radiation, this experiment shows that the rate of emission of the emanation is independent of the pressure of the gas. The effect of pressure on the rate of production of the emanation is discussed in more detail later in section 157. =142. Effect of thickness of layer.= The amount of emanation emitted by a given area of thorium compound depends on the thickness of the layer. With a very thin layer, the current between two parallel plates, placed in a closed vessel as in Fig. 17, is due very largely to the α rays. Since the α radiation is very readily absorbed, the current due to it practically reaches a maximum when the surface of the plate is completely covered by a thin layer of the active material. On the other hand the current produced by the emanation increases until the layer is several millimetres in thickness, and then is not much altered by adding fresh active matter. This falling off of the current after a certain thickness has been reached is to be expected, since the emanation, which takes several minutes to diffuse through the layer above it, has already lost a large proportion of its activity. With a thick layer of thorium oxide in a closed vessel, the current between the plates is largely due to the radiation from the emanation lying between the plates. The following tables illustrate the way in which the current varies with the thickness of paper for both a thin and a thick layer. TABLE I. _Thin Layer._ Thickness of sheets of paper ·0027. No. of Current layers of paper 0 1 1 ·37 2 ·16 3 ·08 TABLE II. _Thick Layer._ Thickness of paper ·008 cm. No. of Current layers of paper 0 1 1 ·74 2 ·74 5 ·72 10 ·67 20 ·55