STORING-UP OF "N" RAYS 41 



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 



