several minutes, this face alone shows activity; an exposure of several hours is necessary for the activity to reach the opposite face.
Aluminium, wood, dry or wet paper, and paraffin do not enjoy the property of storing "N" rays. Calcium sulphide, on the other hand, does possess this property. When I put a few grams of sulphide in an envelope, and then exposed the envelope to "N" rays, I found that its proximity was sufficient to reinforce the phosphorescence of a small mass of previously excited sulphide. This property explains a constant peculiarity that I have previously set forth, viz. that the increase of phosphorescence under the action of "N" rays takes an appreciable time whether to appear or to disappear. For, thanks to the storing-up of the "N" rays, the different parts of a mass of sulphide mutually reinforce their phosphorescence; but since, on the one hand, this reinforcing is progressive, as I have directly proved, and since, on the other hand, the stored-up provision is not immediately exhausted, the result is that when "N" rays are made to fall on phosphorescent calcium
