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Rh action on a small spark. I asked myself if the spark should in this case be considered as an electric phenomenon, or only as producing incandescence like a small gaseous mass. If this latter supposition were correct, the spark could be replaced by a flame. I then produced a quite small flame of gas at the extremity of a metal tube having a very small orifice. This flame was entirely blue. I ascertained that the flame could be used to reveal the presence of "N" rays just like the spark; for when it receives these rays, it becomes whiter and brighter in just the same way. Its variations in glow allowed of four foci being found in a pencil which had passed through a quartz lens; these foci are the same as those detected with the small spark. The small flame behaves therefore, in regard to "N" rays, just like the spark, save that it does not allow of the observation of polarization phenomena.

In order to study more easily the variations in glow, whether of flame or spark, I examine them through a plate of ground glass, about 25 or 30 mms. distant. In this way one obtains,