The cost of fuels for lighting purposes cannot be thoroughly compared throughout a span of years without regard to the fluctuating purchasing power of money, which would be too involved for consideration here. However, it is interesting to make a brief survey throughout the past century. From 1800 until 1845 whale-oil sold for about $.80 per gallon, but after this period it increased in value, owing apparently to its growing scarcity, until it reached a price of $1.75 per gallon in 1855. Fortunately, petroleum was discovered about this time, so that the oil-lamp did not become a luxury. From 1800 to 1850 tallow-candles sold at approximately 20 cents a pound. There being six candles to the pound, and inasmuch as each candle burned about seven hours, the light from a candle cost about 1/2 cent per hour. From 1850 to 1875 tallow-candles sold at an average price of approximately 25 cents a pound. It may be interesting to know that a large match emits about as much light as a burning candle and a so-called safety match about one third as much. A candle-hour is the total amount of light emitted by a standard candle in one hour, and candle-hours in any case are obtained by multiplying the candle-power of the source by the hours of burning. In a similar manner, lumens output multiplied by hours of operation give the lumen-hours. A standard candle may be considered to emit an amount of light approximately equal to 10 lumens. A wax-candle will emit about as much light as a sperm candle but will consume about 10 per cent. less weight of material. A tallow candle will emit about the same amount of light with a consumption about 50 per cent. greater. The tallow-candle has disappeared from use. With the appearance of kerosene distilled from petroleum the camphene lamp came into use. The kerosene cost about 80 cents per gallon during the first few years of its introduction. The price of kerosene averaged about 55 cents a gallon between 1865 and 1875. During the next decade it dropped to about 22 cents a gallon and between 1885 and 1895 it sold as low as 13 cents. Artificial gas in 1865 sold approximately at $2.50 per thousand cubic feet; between 1875 and 1885 at $2.00; between 1885 and 1895 at $1.50.