-sources. Many historical events remain to be touched
upon in later chapters, but it is necessary at this point for the reader
to become acquainted with certain general physical principles in order
that he may read with greater interest some of the chapters which
follow. It is seen that from a standpoint of artificial lighting, the
"dark age" extended well into the nineteenth century. Oil-lamps and
gas-lighting began to be seriously developed at the beginning of the
last century, but the pioneers gave attention chiefly to mechanical
details and somewhat to the chemistry of the fuels. It was not until the
science of physics was applied to light-sources that rapid progress was
made.

All the light-sources used throughout the ages, and nearly all modern
ones, radiate light by virtue of the incandescence of solids or of solid
particles and it is an interesting fact that carbon is generally the
solid which emits light. This is due to various physical characteristics
of carbon, the chief one being its extremely high melting-point.
However, most practicable light-sources of the past and present may be
divided into two general classes: (1) Those in which solids or solid
particles are heated by their own combustion, and (2) those in which the
solids are heated by some other means. Some light-sources include both
principles and some perhaps cannot be included under either principle
without qualification. The luminous flames of burning material such as
those of wood-splinters, candles, oil-lamps, and gas-jets, and the
glowing embers of burning material appear in the first class; and
incandescent gas-mantles, electric filaments, and arc-lamps to some
extent are representative of the second class. Certain "flaming" arcs
involve both principles, but the light of the firefly, phosphorescence,
and incandescent gas in "vacuum" tubes cannot be included in this
simplified classification. The status of these will become clear later.

It has been seen that flames have been prominent sources of artificial
light; and although of low luminous efficiency, they still have much to
commend them from the standpoints of portability, convenience, and
subdivision. The materials which have been burned for light, whether
solid or liquid, are rich in carbon, and the solid particles of carbon
by virtue of their incandescence are responsible for the brightness of a
flame. A jet of pure hydrogen gas will burn very hot but with so low a
brightness as to be