With yttria in a vacuum tube, the point of maximum phosphorescence, as I have already pointed out, lies on the margin of the dark space.  The diagram (Fig. 24) shows approximately the degree of phosphorescence in different parts of a tube at an internal pressure of 0.25 millimeter, or 330 M. On the top you see the positive and negative poles, A and B, the latter having the outline of the dark space shown by a dotted line, C. The curve, D E F, shows the relative intensities of the phosphorescence at different distances from the negative pole, and the position inside the dark space at which phosphorescence does not occur.  The height of the curve represents the degree of phosphorescence.  The most decisive effects of phosphorescence are reached by making the tube so large that the walls are outside the dark space, while the material submitted to experiment is placed just at the edge of the dark space.

Hitherto I have spoken only of the phosphorescence of substances placed under the negative pole.  But from numerous experiments I find that bodies will phosphoresce in actual contact with the negative pole.

[Illustration:  FIG. 24—­PRESSURE = 0.25 MM. = 330 M.]

This is only a temporary phenomenon, and ceases entirely when the exhaustion is pushed to a very high point.  The experiment is one scarcely possible to exhibit to an audience, so I must content myself with describing it.  A U-tube, shown in Fig. 25, has a flat aluminum pole, in the form of a disk, at each end, both coated with a paint of phosphorescent yttria.  As the rarefaction approaches about 0.5 millimeter the surface of the negative pole, A, becomes faintly phosphorescent.  On continuing the exhaustion this luminosity rapidly diminishes, not only in intensity but in extent, contracting more and more from the edge of the disk, until ultimately it is visible only as a bright spot in the center.  This fact does not prop a recent theory, that as the exhaustion gets higher the discharge leaves the center of the pole and takes place only between the edge and the walls of the tube.

[Illustration:  FIG. 25.]

If the exhaustion is further pushed, then, at the point where the surface of the negative pole ceases to be luminous, the material on the positive pole, B, commences to phosphoresce, increasing in intensity until the tube refuses to conduct, its greatest brilliancy being just short of this degree of exhaustion.  The probable explanation is that the vagrant molecules I introduce in the next experiment, happening to come within the sphere of influence of the positive pole, rush violently to it, and excite phosphorescence in the yttria, while losing their negative charge.

* * * * *

[Continued from SUPPLEMENT, No. 794, page 12690.]

GASEOUS ILLUMINANTS.[1]

[Footnote 1:  Lectures recently delivered before the Society of Arts, London.  From the Journal of the Society.]