marked effect was obtained by interposing the quartz plate between the source and the sulphide, quite close to the latter.
In these experiments, the secondary emission by the quartz is added to the "N" rays directly emanating from the source. This secondary emission has, indeed, its origin in the whole mass of the quartz, and not at the surface only, for if several plates of quartz be successively placed on top of each other, the effect is seen to increase with each added plate. Iceland spar, fluor spar, barite, glass, etc., behave like quartz. The filament of a Nernst lamp remains active for several hours after the lamp is extinguished.
A piece of gold, laterally brought near to the sulphide while it is being subjected to "N" rays, increases its glow (note 10); lead, platinum, silver, zinc, etc., produce the same effects. These actions persist after the extinction of "N" rays, as in the case of quartz. Nevertheless, the property of secondary ray emission only permeates slowly through a metallic mass. Thus, if one of the faces of a sheet of lead 2 mms. thick has been exposed to "N" rays for
