That was over seventy years ago. What a change has been made within these seventy years! In cities and large towns almost every new house is piped for gas. Gas companies are formed for supplying this illuminating product to the inhabitants. Gas meters have been perfected which measure the quantity of gas, so that one pays for no more than he uses. Moreover, the towns and cities put up street lights which burn this same gas in the night, making it easy, convenient, and safe to traverse the streets at any hour. Bituminous or soft coal is used in the manufacture of illuminating gas, as anthracite contains less of the needed materials. Gases are easily driven off from bituminous coal whenever it is heated, if air is kept from it. At the works, therefore, the coal is placed in large closed ovens, called _retorts_. These are directly over furnace fires, which are kept vigorously burning. The gases pass out of the coal and, rising, enter a series of long pipes. The coal which is left in the retorts is called coke. This process is called _distillation_. Many substances pass off with the gas, from which it must be cleaned. Tar and ammonia become liquids when cooled, and are left behind as the gas passes through cold water. The series of iron pipes in which this process is carried on is called the _condenser_. Then the gas is carried through the _purifier_, in which all other impurities are removed. When thoroughly purified the gas passes into the _gasometer_. This usually consists of two round iron cylinders of nearly the same size, one inside of the other. The outside cylinder has no roof; the inside has no floor. The sides of the inner one go down into a trench filled with water. Its top is held up by the gas, which comes into it from the purifier. [Illustration: A GASOMETER.] The roof of the inner cylinder presses down heavily upon the gas, pushing it into the large _main pipes_, which run from the gasometer through the principal streets. Smaller mains connect with these and the gas is pushed into the _service pipes_, which enter the houses. When a stopcock is opened in any house the pressure of the gasometer pushes the gas through, it may be, miles of pipes, and out through the burner, where it may be lighted. Many houses have a simple electric-lighting attachment, so that by merely turning a stopcock the gas is turned on and by pulling a chain an electric spark sets the gas on fire, flooding the room with light. Within a few years illuminating gas has greatly diminished in price. It costs a little more than kerosene, but it is more convenient in many ways. The danger of carrying lamps from room to room is avoided, as well as the disagreeable task of filling them. Still the gas flame is less steady than that of the kerosene lamp, and is therefore less serviceable for reading. For the poor man the kerosene light is a great blessing, while for all who can afford the extra cost the gaslight is a greater convenience. CHAPTER VI.