and the extra motion 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