duction on a
somewhat more conservative basis.

At the present time the light-source of chief interest in electric
lighting is the incandescent filament lamp; but its luminous efficiency
is limited, as has been shown in a previous chapter. When light is
emitted by virtue of its temperature much invisible radiant energy
accompanies the visible energy. The highest luminous efficiency
attainable by pure temperature radiation will be reached when the
temperature of a normal radiator reaches the vicinity of 10,000 deg.F. to
11,000 deg.F. The melting-points of metals are much lower than this. The
tungsten filament in the most efficient lamps employing it is operating
near its melting-point at the present time. Carbon is a most attractive
element in respect to melting-point, for it melts at a temperature
between 6000 deg.F. and 7000 deg.F. Even this is far below the most efficient
temperature for the production of light by means of pure temperature
radiation. There are possibilities of higher efficiency being obtained
by operating arcs or filaments under pressure; however, it appears that
highly efficient light of the future will result from a radical
departure.

Scientists are becoming more and more intimate with the structure of
matter. They are learning secrets every year which apparently are
leading to a fundamental knowledge of the subject. When these mysteries
are solved, who can say that man will not be able to create elements to
suit his needs, or at least to alter the properties of the elements
already available? If he could so alter the mechanism of radiation that
a hot metal would radiate no invisible energy, he would have made a
tremendous stride even in the production of light by virtue of high
temperature. This property of selective radiation is possessed by some
elements to a slight degree, but if treatment could enhance this
property, luminous efficiency would be greatly increased. Certainly the
principle of selectivity is a byway of possibilities.

A careful study of commonplace factors may result in a great step in
light-production without the creation of new elements or compounds, just
as such a procedure doubled the luminous efficiency of the tungsten
filament when the gas-filled lamp appeared. There are a few elements
still missing, according to the Periodic Law which has been so valuable
in chemistry. When these turn up, they may be found to possess valuable
properties for light-production; but this is