the consequences of the use of gas were appalling.  But this protest failed and in 1835 a gas-plant was founded in Philadelphia.  Thus gas-lighting, which to Sir Walter Scott was a “pestilential innovation” projected by a madman, weathered its early difficulties and grew to be a mighty industry.  Continued improvements 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 in commercial use.  His first mantles were unsatisfactory, but they were improved in 1886 by the use of thoria, an oxide of thorium, in conjunction with other rare-earth oxides.  His mantle was now not only stronger but it gave more light.  Later he greatly improved the mantles by purifying the oxides and finally achieved his great triumph by adding a slight amount of ceria, an oxide of cerium.  Welsbach is deserving of a great deal of credit for his extensive work, which overcame many difficulties and finally gave to the world a durable mantle that greatly increased the amount of light previously obtainable from gas.