f producing light has an advantage in the
selection of a gas among a large number of possibilities, and some of
the color effects of the future may be obtained by means of it. Claude
has lately worked on light-production by vacuum tubes and has combined
the neon tube with the mercury-vapor tube. The spectrum of neon to a
large extent compensates for the absence of red light in the mercury
spectrum, with a result that the mixture produces a more satisfactory
light than that of either tube. However, this mode of light-production
has not proved practicable in its present state of development.
Fundamentally the limitations are those of the inherent spectral
characteristics of gases. Doubtless the possibilities of the mechanisms
of the tubes and of combining various gases have not been exhausted.
Furthermore, if man ever becomes capable of controlling to some extent
the structure of elements and of compounds, this method of
light-production is perhaps more promising than others of the present
day.

There is another attractive method of producing light and it has not
escaped the writer of fiction. H. G. Wells, with his rare skill and with
the license so often envied by the writer of facts, has drawn upon the
characteristics of fluorescence and phosphorescence. In his story "The
First Men in the Moon," the inhabitants of the moon illuminate their
caverns by utilizing this phenomenon. A fluorescent liquid was prepared
in large quantities. It emitted a brilliant phosphorescent glow and when
it splashed on the feet of the earth-men it felt cold, but it glowed for
a long time. This is a possibility of the future and many have had
visions of such lighting. If the ceiling of a coal-mine was lined with
glowing fireflies or with phosphorescent material excited in some
manner, it would be possible to see fairly well with the dark-adapted
eyes.

This leads to the class of phenomena included under the general term
"luminescence." The definition of this term is not thoroughly agreed
upon, but light produced in this manner does not depend upon temperature
in the sense that a glowing tungsten filament emits light because it is
sufficiently hot. A phosphorus match rubbed in the moist palm of the
hand is seen to glow, although it is at an ordinary temperature. This
may be termed "chemi-luminescence." Sidot blende, Balmain's paint, and
many other compounds, when illuminated with ordinary light, and
especially with ultra-violet and violet ray