The discovery of petroleum helped further to light the world and distinguish the century. It gave us gasolene, naphtha, gas oil, astral oil, and the very effective “mineral sperm,” which is almost universally used in lighthouses and as headlights for locomotives. With the addition of kerosene, a favorite light of the beginning of the century—the tallow dip of our grandmothers—began to fall into disuse. The homelike pictures of housewives at their annual candle-dippings, or in the manipulation of their moulds, became venerable antiques. Candle-light paled in the presence of the higher illuminants. Though still a convenient light under certain circumstances, it plays a gradually diminishing part amid its superiors. One of the signal triumphs of the century has been the introduction of gas-lighting. Though illuminating gas made from coal was known as early as 1691, it did not come into use, except for experiments or in a very special way, until the beginning of the nineteenth century. In 1809, a few street lamps were lit with gas in London. An unsuccessful attempt was made to introduce gas into Baltimore in 1821. Between 1822 and 1827, the gas-light began to have a feeble foothold in Boston and New York. Other cities began to introduce it as an illuminant in streets and, eventually, in houses. But the process was very slow, owing to intense opposition on the part of both savants and common people, who saw in it a sure means of destruction by poison, explosion, or fire. It was not much before the middle of the century that prejudice against illuminating gas was sufficiently allayed to admit of its general use. But meanwhile many valuable experiments as to its production and adaptation were going on. The most productive source of illuminating gas was found to be bituminous coal. Though gas could be produced by distillation from other substances, such as shale, lignite, petroleum, water, turf, resins, oils, and fats, none could compete in quality, quantity, and economy with what is known as ordinary coal gas, at least, not until the time came, quite late in the century, when it was found that non-luminous gases, such as water gas, could be rendered luminous by impregnating them with hydrocarbon vapor. This became known commercially as water gas, and it is now largely used in place of coal gas, because it is cheaper and, for the most part, equally effective as a luminant.