r'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 no