plate, with a photo-electric cell, and by
other means. Their spectral lines or bands also extend into the
infra-red region and here they are studied by means of the bolometer or
other apparatus for detecting radiant energy by the heat which it
produces upon being absorbed. Spectrum analysis is far more sensitive
than the finest weighing balance, for if a grain of salt be dissolved in
a barrel of water and an asbestos strip be soaked in the water and held
in a Bunsen flame, the yellow color characteristic of sodium will be
detectable. A wonderful example of the possibilities of this method is
the discovery of helium in the sun before it was found on earth! Its
spectral lines were detected in the sun's spectrum and could not be
accounted for by any known element. However, it should be stated that
the spectrum of an element differs generally with the manner obtained.
The electric spark, the arc, the electric discharge in a vacuum tube,
and the flame are the means usually employed.

The spectrum has been dwelt upon at some length because it is of great
importance in light-production and probably will figure strongly in
future developments. Although in lighting little use has been made of
the injection of chemical salts into ordinary flames, it appears certain
that such developments would have risen if electric illuminants had not
entered the field. However, the principle has been applied with great
success in arc-lamps. In the first arc-lamps plain carbon electrodes
were used, but in some of the latest carbon-arcs, electrodes of carbon
impregnated with various salts are employed. For example, calcium
fluoride gives a brilliant yellow light when used in the carbons of the
"flame" arc. These are described in detail later.

Following this principle of light-production the vacuum tubes were
developed. Crookes studied the light from various gases by enclosing
them in a tube which was pumped out until a low vacuum was produced. On
connecting a high voltage to electrodes in each end, an electrical
discharge passed through the residual gas making it luminous. The
different gases show their characteristic spectra and their desirability
as light-producers is at once evident.

However, the most general principle of light-production at the present
time is the radiation of bodies by virtue of their temperature. If a
piece of wire be heated by electricity, it will become very hot before
it becomes luminous. At this temperature it is e