tive 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.]

By Prof. VIVIAN B. LEWES.

V.

Having now brought before you the various methods by which ordinary
coal gas can be enriched, so as to give an increased luminosity to the
flame, I wish now to discuss the methods by which the gas can be
burnt, in order to yield the greatest amount of light, and also the
compounds which are produced during combustion.

In the first lecture, while discussing the theory of luminous flames,
I pointed out that, in an atmospheric burner, it was not the oxygen of
the air introduced combining with and burning up the hydrocarbons, and
so preventing the separation of incandescent carbon, which gave the
non-luminous flame, but the diluting action of the nitrogen, which
acted by increasing the temperature at which the hydrocarbons are
broken up, and carbon liberated, a fact which was proved by
observation that heating the mixture of gas and air again restored the
luminosity of the flame. This experiment clearly shows that
temperature is a most important factor in the illuminating value of a
flame, and this is still further shown by a study of the action of the
diluents present in coal gas, the non-combustible ones