ts and increasing output not only altered the course
of civilization by increased and adequate lighting but they reduced the
cost of lighting over the span of the nineteenth century to a small
fraction of its initial cost.

Think of the city of Philadelphia in 1800, with a population of about
fifty thousand, dependent for its lighting wholly upon candles and
oil-lamps! Washington's birthday anniversary was celebrated in 1817 with
a grand ball attended by five hundred of the elite. An old report of the
occasion states that the room was lighted by two thousand wax-candles.
The cost of this lighting was a hundred times the cost of as much light
for a similar occasion at the present time. Can one imagine the present
complex activities of a city like Philadelphia with nearly two million
inhabitants to exist under the lighting conditions of a century ago?
To-day there are more than fifty thousand street lamps in the city--one
for each inhabitant of a century ago. Of these street lamps about
twenty-five thousand burn gas. This single instance is representative of
gas-lighting which initiated the "light age" and nursed it through the
vicissitudes of youth. The consumption of gas has grown in the United
States during this time to three billion cubic feet per day. For
strictly illuminating purposes in 1910 nearly one hundred billion cubic
feet were used. This country has been blessed with large supplies of
natural gas; but as this fails new oil-fields are constantly being
discovered, so that as far as raw materials are concerned the future of
gas-lighting is assured for a long time to come.

The advent of the gas-mantle is responsible for the survival of
gas-lighting, because when it appeared electric lamps had already been
invented. These were destined to become the formidable light-sources of
the approaching century and without the gas-mantle gas-lighting would
not have prospered. Auer von Welsbach was conducting a spectroscopic
study of the rare-earths when he was confronted with the problem of
heating these substances. He immersed cotton in solutions of these salts
as a variation of the regular means for studying elements by injecting
them into flames. After burning the cotton he found that he had a
replica of the original fabric composed of the oxide of the metal, and
this glowed brilliantly when left in the flame.

This gave him the idea of producing a mantle for illuminating purposes
and in 1885 he placed such a mantle i