extent in the Holliday lamp.  In this lamp the oil is contained in a reservoir from the bottom of which a fine metal tube carries the oil down to a rose-burner.  The oil is heated by the flame and the vaporized mineral oil which escapes through small orifices is burned.  This type of lamp has undergone many physical changes, but its principle survives to the present time in the gasolene and kerosene burners hanging on a pole by the side of the street-peddler’s stand.

Although petroleum products were not used to any appreciable extent for illuminating-purposes until after the middle of the nineteenth century, mineral oil is mentioned by Herodotus and other early writers.  In 1847 petroleum was discovered in a coal-mine in England, but the supply failed in a short time.  However, the discoverer, James Young, had found that this oil was valuable as a lubricant and upon the failure of this source he began experiments in distilling oil from shale found in coal deposits.  These were destined to form the corner-stone of the oil industry in Scotland.  In 1850 he began producing petroleum in this manner, but it was not seriously considered for illuminating-purposes.  However, in Germany about this time lamps were developed for burning the lighter distillates and these were introduced into several countries.  But the price of these lighter oils was so great that little progress was made until, in 1859, Col.  E. L. Drake discovered oil in Pennsylvania.  By studying the geological formations and concluding that oil should be obtained by boring, Drake gave to the world a means of obtaining petroleum, and in quantities which were destined to reduce the price of mineral oil to a level undreamed of theretofore.  To his imagination, which saw vast reservoirs of oil in the depths of the earth, the world owes a great debt.  Lamps were imported from Germany to all parts of the civilized world and the kerosene lamp became the prevailing light-source.  Hundreds of American patents were allowed for oil-lamps and their improvements in the next decade.

[Illustration:  Lamps of A century or two ago]

[Illustration:  Elaborate fixtures of the age of candles]

The crude petroleum, of course, is not fit for illuminating purposes, but it contains components which are satisfactory.  The various components are sorted out by fractional distillation and the oil for burning in lamps is selected according to its volatility, viscosity, stability, etc.  It must not be so volatile as to have a dangerously low flashing-point, nor so stable as to hinder its burning well.  In this fractional distillation a vast variety of products are now obtained.  Gasolene is among the lighter products, with a density of about 0.65; kerosene has a density of about 0.80; the lubricating-oils from 0.85 to 0.95; and there are many solids such as vaseline and paraffin which are widely used for many purposes.  This process of refining oils is now the source of paraffin for making candles, in which it is usually mixed with substances like stearin in order to raise its melting-point.