common flat gas burner of today was discovered in 1820. Luminosity of a coal gas flame is determined by the temperature to which the carbon particles are raised. To make use of this principle the flames of some burners were made to impinge upon each other and rise as a flat sheet of flame. In another, the regenerative principle of heating the air before bringing it to the flame was used. Another scientific improvement was that of using a nonconducting burner tip to take the place of the usual iron one and thus prevent the heat from being conducted through the iron instead of heating the carbon particles.

By 1880 the gas burners were 10 to 15 times as efficient as the first one made by Murdoch, but the gas mantle, which is by far the most efficient source of light under average conditions today, was still unknown. The idea of using an unburnable refractory substance heated to a high temperature in a flame to produce a bright light was known as early as 1826 when experimenters heated lime to a high temperature by the oxy-hydrogen blowpipe. This idea, when using lime or magnesia or platinum, was tried out several times later but no practical light resulted until Welsbach made his discovery.

An important preliminary to the Welsbach mantle was made by Bunsen about 1855 when he devised a burner for laboratory use in which the gas and the air were so mixed before burning that very complete combustion was secured. The flame was not luminous, however. It is essential that the combustion of hydrocarbons be complete in a gas mantle else there will be a deposit of soot to render the mantle ineffective. Bunsen's scientific burner finds use both for lighting and for heating purposes.

Welsbach's discovery of the gas mantle was made almost by accident. He was experimenting in Professor Bunsen's laboratory with the rare earths by use of the spectroscope and after he had dipped cotton in a solution of the earths and burned the cotton away in a flame, he

discovered that the oxides of the earths held the shape of the cotton and glowed brilliantly in the flame. This set his imagination going and led him to commence rather elaborate experiments with fibers that had been soaked in solutions of rare earths. His teacher, Professor Bunsen, tried to discourage this line of research, saying that it could lead to nothing of value, but Welsbach continued his investigations in spite of discouragements and finally made the discovery that has meant so much to better illumination and at the same time made him very wealthy.

His tests with the different oxides of the earths to discover which were the best, led to the development of the thoria mantle. It required considerable research after the thoria mantle was patented in 1886 before it was learned that if the thoria had one per cent of ceria mixed with it the mantle would give an excellent light, whereas it is unsatisfactory when only the thoria is used.

With good earth oxides for making the mantle a new series of tests were necessary in order that a more satisfactory fiber than cotton might be found for absorbing the earths in making the mantle. Artificial silk, chemically treated ramie fiber, and cotton were finally found most suitable.

The Bunsen burner was subjected to consideral experimental work in adopting it to the mantles and in making it work in the inverted position found in most mantles today. These experiments have improved the gas mantle until it is perhaps the most efficient light in use under average conditions today. It is 30 to 35 times as efficient as Murdoch's first gas light. According to the U. S. Bureau of Standards if the cost of gas is $1 per 1000 cubic feet, electricity 10 cents per kilowatt hour, candles 12 cents a pound, and kerosene 15 cents a gallon, the relative costs of the gas mantle, tungsten electric light, candle and kerosene lamp would be 5 cents, 122 cents, 200 cents

and 20 cents respectively for 1000 candle-hours of light. The kerosene mantle would cost about the same as the gas mantle, but the former has not yet become practical in operation.

Besides coal gas, which is mainly referred to above, scientific workers have produced several others. Water gas, which forms over half of the ordinary city gas supply, is by far the most important, although the recently developed oil gases, acetylene, methan-hydrogen, gasoline and other gases are important under certain conditions. Practically all of the gases have been developed, improved, and tested by the aid of science. The commercial production of such gases as acetylene, which could not be made except in quantities sufficient for laboratory use before the electric furnace was well developed, are obviously productions of research laboratories.

CHAPTER XX

THE STORY OF LIGHTING WITH ELECTRICITY

In spite of the fact that light can be produced somewhat more cheaply under average conditions by gas mantles than by the present types of electric lights, the use of electricity has advantages which are rapidly making it the one great source of artificial light. Besides the smaller breakage of accessories, the elimination of the smell of gas and the danger of asphyxia or gas explosions, electricity is better adapted for use in sparsely settled districts. Also the electric wiring gives a ready source of power for operating machines in the home or the factory. Over 40 per cent of all homes (8,467,000 domestic light customers) in the United States are supplied with electricity and the number is rapidly increasing. Of the 320 billion cubic feet of gas produced in the United States about 24 billion are used to generate electricity. The coal consumption for electricity is 32 million tons, while for gas it is only 9 million. Hydro-electric power is used to generate 15 billion of the 41 billion kilowatt-hours of electricity in use today. One of the reasons why electric power is relatively more important in the sparsely settled territories than in the great centers of population where coal is cheap, is that hydro-electric power is available.

The introduction of electricity for illumination had its inception in 1801 when Sir Humphry Davy began his experiments on electric arc lights. But Davy, as well as all experimenters working before about 1860, had only chemical battery electricity to work with, so their success was limited. The period between Davy's discovery and