When scientists learned by exploring flames that brightness was due to the radiation of light by incandescent solid matter, the way was open for many experiments.  In the early days of gas-lighting investigations were made to determine the relation of illuminating value to the chemical constitution of the gas.  The results combined with a knowledge of the necessity for solid carbon in the flame led to improvements in the gas for lighting purposes.  Gas rich in hydrocarbons which in turn are rich in carbon is high in illuminating value.  Heating-effect depends upon heat-units, so the rating of gas in calories or other heat-units per cubic foot is wholly satisfactory only for gas used for heating.  The chemical constitution is a better indicator of illuminating value.

As scientific knowledge increased, efforts were made to get solid matter into the flames of light-sources.  Instead of confining efforts to the carbon content of the gas, solid materials were actually placed in the flame, and in this manner various incandescent burners were developed.  A piece of lime placed in a hydrogen flame or that of a Bunsen burner is seen to become hot and to glow brilliantly.  By producing a hotter flame by means of the blowpipe, in which hydrogen and oxygen are consumed, the piece of lime was raised to a higher temperature and a more intense light was obtained.  In Paris there was a serious attempt at street-lighting by the use of buttons of zirconia heated in an oxygen-coal-gas flame, but it proved unsuccessful owing to the rapid deterioration of the buttons.  This was the line of experimentation which led to the development of the lime-light.  The incandescent burner was widely employed, and until the use of electricity became common the lime-light was the mainstay for the stage and for the projection of lantern slides.  It is in use even to-day for some purposes.  The origin of the phrase “in the lime-light” is obvious.  The luminous intensity of the oxyhydrogen lime-light as used in practice was generally from 200 to 400 candle-power.  The light decreases rapidly as the burner is used, if a new surface of lime is not presented to the flame from time to time.  At the high temperatures the lime is somewhat volatile and the surface seems to change in radiating power.  Zirconium oxide has been found to serve better than lime.

Improvements were made in gas-burners in order to obtain hotter flames into which solid matter could be introduced to obtain bright light.  Many materials were used, but obviously they were limited to those of a fairly high melting-point.  Lime, magnesia, zirconia, and similar oxides were used successfully.  If the reader would care to try an experiment in verification of this simple principle, let him take a piece of magnesium ribbon such as is used in lighting for photography and ignite it in a Bunsen flame.  If it is held carefully while burning, a ribbon of ash (magnesia) will be obtained intact.  Placing this in the faintly