tion due to the electrical impetus.

Experiments show that in such tubes a few molecules may traverse more
than a hundred times the _mean_ free path, with a correspondingly
increased velocity, until they are arrested by collisions. Indeed, the
molecular free path may vary in one and the same tube, and at one and
the same degree of exhaustion.

Very many bodies, such as ruby, diamond, emerald, alumina, yttria,
samaria, and a large class of earthy oxides and sulphides,
phosphoresce in vacuum tubes when placed in the path of the stream of
electrified molecules proceeding from the negative pole. The
composition of the gaseous residue present does not affect
phosphorescence; thus, the earth yttria phosphoresces well in the
residual vacua of atmospherical air, of oxygen, nitrogen, carbonic
anhydride, hydrogen, iodine, sulphur and mercury.

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 nega