Turning again to flames, another interesting physical phenomenon is seen on placing solutions of different chemical salts in the flame.  For example, if a piece of asbestos is soaked in sodium chloride (common salt) and is placed in a Bunsen flame, the pale-blue flame suddenly becomes luminous and of a yellow color.  If this is repeated with other salts, a characteristic color will be noted in each case.  The yellow flame is characteristic of sodium and if it is examined by means of a spectroscope, a brilliant yellow line (in fact, a double line) will be seen.  This forms the basis of spectrum analysis as applied in chemistry.

Every element has its characteristic spectrum consisting usually of lines, but the complexity varies with the elements.  The spectra of elements also exhibit lines in the ultra-violet region which may be studied with a photographic 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.