characteristic spectrum in an electric discharge, but that it is
driven with extraordinary rapidity out of the course of the
discharge.]

Here is a tube (Fig. 15, P=0.00068 millimeter, or 0.9 M), with two
negative electrodes, AA', so placed as to protect two luminous spots
on the phosphorescent glass of the tube. One electrode, A', is of pure
silver, a volatile metal; the other, A, is of aluminum, practically
non-volatile. A quantity of "electrode matter" will be shot off from
the silver pole, and practically none from the aluminum pole; but you
see that in each case the phosphorescence, CC', is identical. Had the
radiant electrode matter been the active agent, the more intense
phosphorescence would proceed from the more volatile pole.

A drawing of another experimental piece of apparatus is shown in Fig.
16. A pear-shaped bulb of German glass has near the small end an inner
concave negative pole, A, of pure silver, so mounted that its
inverted image is thrown upon the opposite end of the tube. In front
of this pole is a screen of mica, C, having a small hole in the
center, so that only a narrow pencil of rays from the silver pole can
pass through, forming a bright spot, D, at the far end of the bulb.
The exhaustion is about the same as in the previous tube, and the
current has been allowed to pass continuously for many hours so as to
drive off a certain portion of the silver electrode; and upon
examination it is found that the silver has all been deposited in the
immediate neighborhood of the pole; while the spot, D, at the far end
of the tube, that has been continuously glowing with phosphorescent
light, is practically free from silver.

[Illustration: FIG. 15.--PRESSURE = 0.00068 MM. = 0.9 M.]