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 rays, will continue to glow for a long time.  Despite their brightness they will be cold to the touch.  This phenomenon would be termed “photo-luminescence,” although it is better known as “phosphorescence.”  It should be noted that the latter term was carelessly originated, for phosphorus has nothing to do with it.  The glow of the Geissler tube or electrically excited gas at low pressure would be an example of “electro-luminescence.”  The luminosity of various salts in the Bunsen-flame is due to so-called luminescence and there are many other examples of light-production which are included in the same general class.  Inasmuch as light is emitted from comparatively cold bodies in these cases, it is popularly known as “cold” light.

There are many instances of light being emitted without being accompanied by appreciable amounts of invisible radiant energy and it is natural to hope for practical possibilities in this direction.  As yet little is known regarding the efficiency of light-production by phosphorescence.  The luminous efficiency of the radiant energy emitted by phosphorescent substances has been studied, but it seems strange that among the vast works on phosphorescent phenomena, scarcely any mention is made of the efficiency of producing light in this manner.  For example, assume that phosphorescent zinc sulphide is excited by the light from a mercury-arc.  All the energy falling upon it is not capable of exciting phosphorescence, as may be readily shown.  Assuming that a known amount of radiant energy of a certain wave-length has been permitted to fall upon the phosphorescent material, then in the dark the latter may be seen to glow for a long time.  An interesting point to investigate is the relation of the output to input; that is, the ratio of the total emitted light to the total exciting energy.  This is a neglected aspect in the study of light-production by this means.

The firefly has been praised far and wide as the ideal light-source.  It is an efficient radiator of light, for its light is “cold”; that is, it does not appear to be accompanied by invisible radiant energy.  But little is said about its efficiency as a light-producer.  Who knows how much fuel its lighting-plant consumes?  The chemistry of light-production by living organisms is being unraveled and this part of the phenomenon will likely be laid bare before long.  For an equal amount of energy radiated, the firefly emits a great many times more light than the most efficient lamp in use at the present time, but before the firefly is pronounced ideal, the efficiency of its light-producing process must be known.