The invention of the electric light belongs entirely to the nineteenth century. Sir Humphry Davy was the first to produce it. He had a great battery of two thousand cells at the Royal Institution, and he found that when he joined the two ends of the battery by wires and separated the wires again, a vivid light played between the two ends of the wires. This light or flame was so hot that it melted all metals, including platinum, which is extremely difficult to melt. He tried to find some substance which would not melt, but would yet conduct electricity. He tried charcoal, but found it conducted the current very badly. He plunged it in mercury, and so obtained a fairly good conductor, which did not melt in the electric flame, but burned away rather quickly. However, he showed this light at the Royal Institution so long ago as the year 1808, and it excited great wonder and curiosity. For many years this remained the only experiment, and it was only towards the middle of the century that a substance was found which burned away very slowly. That substance was discovered at the bottom of the crucibles used for making coal gas. When all the coal gas had been driven out of the coal by heat, and only the coke remained, it was found that a very hard and compact black mass remained at the bottom of the crucible. That substance was called “gas carbon.” It conducted electricity well, and made a very good substance for experiments on the electric flame. It was cut into small rods, and the flame was made to play between the ends of two such rods. The flame was always seen to be in the form of an arch or arc, curving upwards between the two rods. This was due to the heat of the flame which produced an upward current of air between the pencils. The flame was therefore called the “electric arc,” and a lamp containing it was called an “arc lamp.” Such arc lamps are used to the present day for lighting streets and squares. One such lamp is as bright as several thousand candles burning together. The light which comes from the white hot ends of the carbons is the brightest that it is possible to produce artificially. But the arc lamp is really too bright for ordinary use. It is quite blinding, and it would not do to bring it into an ordinary room. For many years electricians were busy trying to make another kind of electric lamp which would not give such a glaring light as the electric arc. The two men who did most to solve this problem were Thomas Edison in the United States of America and Joseph Swan in England. Edison was born in the year 1847. He never went to school. All the education he ever got was given to him by his mother. [Illustration: Fig. 64.--Thomas Edison.]