energy in factory, furnace, forge, and rolling-mill. In fact, no marvel of the century has been at once so weird and inscrutable in its origin as natural gas, or more potential as an agency within the areas to which its use is limited. The question is ever uppermost in connection with natural gas, will it last? The gas springs of the Caucasus Mountains have been burning for centuries. But that is where nature’s internal forces have their correlations and compensations. Where it is quite otherwise, that is, where the vents of natural gas reservoirs are abnormally numerous, or where those reservoirs are drained to the extreme for commercial purposes, not to say through sheer wastefulness, the geologist is ready to surmise that the natural gas supply cannot be a perpetual one. But one of the most magnificent triumphs of the century in the matter of light came about through the agency of electricity. We have already seen the beginnings of electric lighting in the discovery of Sir Humphrey Davy, in 1809, that when the ends of two conducting wires, mounted with charcoal pieces, were brought close together, a brilliant light, in the shape of an arc or curve, leaped from one piece of charcoal to the other. Davy’s charcoal pieces or carbons were consumed by the fierce heat evolved; but the principle was established that an electric current, so interrupted, was a vivid light-producer, and might be made permanently so if a substance capable of resisting the heat could be substituted for his charcoal tips, and a generator of electricity of sufficient power and economy in use could be substituted for his voltaic batteries or cells. Upon these two essentials hung the future of the electric light. The first essential, that of a substance at the ends of the wires or in the midst of the electric circuit which would resist the heat, was soon met by the use of specially prepared and hard graphite carbon tips, in the shape of candles. But the second essential, a generator of electricity cheaper and more powerful than the voltaic cell, was not met with till the dynamo machine reached an advanced stage of perfection; that is, about 1867. [Illustration: ELECTRIC ARC LIGHT.]