these are intended merely to serve as examples of the difficulties encountered.  Doubtless, similar controversies arose in other countries in which pioneers were also nursing gas-lighting to maturity.  However, it is certain that much of the early progress of lighting of this character was fathered in England.  Gas-lighting was destined to become a thriving industry, and is of such importance in lighting that another chapter is given its modern developments.

VII

THE SCIENCE OF LIGHT-PRODUCTION

In previous chapters much of the historical development of artificial lighting has been presented and several subjects have been traced to the modern period which marks the beginning of an intensive attack by scientists upon the problems pertaining to the production of efficient and adequate light-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.