The variation of luminosity with different distances between the screens is to be expected. The amount of excited activity deposited on the boundaries is proportional to the amount of emanation present. Since the emanation is equally distributed, the amount of excited activity deposited on the screens, due to the emanation between them, varies directly as the distance, provided the distance between the screens is small compared with their dimensions. Such a result would also follow if the phosphorescence were due to the radiation from the emanation itself, provided that the pressure of the gas was low enough to prevent absorption of the radiation from the emanation in the gas itself between the screens. Measurements of Emanating Power. =149. Emanating power.= The compounds of thorium in the solid state vary very widely in the amount of emanation they emit under ordinary conditions. It is convenient to use the term _emanating power_ to express the amount of emanation given off per second by one gram of the compound. Since, however, we have no means of determining absolutely the amount of emanation present, all measurements of emanating power are of necessity comparative. In most cases, it is convenient to take a given weight of a thorium compound, kept under conditions as nearly as possible constant, and to compare the amount of emanation of the compound to be examined with this standard. In this way comparisons of the emanating power of thorium compounds have been made by Rutherford and Soddy[244], using an apparatus similar to that shown in Fig. 51 on page 240. A known weight of the substance to be tested was spread on a shallow dish, placed in the glass tube _C_. A stream of dry dust-free air, kept constant during all the experiments, was passed over the compound and carried the emanation into the testing vessel. After ten minutes interval, the current due to the emanation in the testing vessel reached a constant value. The compound was then removed, and the standard comparison sample of equal weight substituted; the saturation current was observed when a steady state was again reached. The ratio of these two currents gives the ratio of the emanating power of the two samples. It was found experimentally that, for the velocities of air current employed, the saturation current in the testing vessel was directly proportional to the weight of thorium, for weights up to 20 grams. This is explained by the supposition that the emanation is removed by the current of air from the mass of the compound, as fast as it is formed. Let _i₁_ = saturation current due to a weight ω₁ of the standard, _i₂_ = „ „ „ „ ω₂ of the sample to be tested. (emanating power of specimen) _i₂_ ω₁ Then ---------------------------- = ---- --- (emanating power of standard) _i₁_ ω₂ By means of this relation the emanating power of compounds which are not of equal weight can be compared.