d 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