When soft coal is heated in a closed receptacle a gas is formed which will burn. To show this we have only to take an ordinary clay pipe, put a little piece of coal in the bowl, close the top with wet clay, and put the bowl part of the pipe in the fire. When it is quite hot, a gas will be found coming out of the stem of the pipe, which will, when lighted, burn. The Story In a Gas Jet. ~HOW ILLUMINATING GAS IS MADE~ Soft coal is heated in large tubes of fire clay called retorts, and the gas that is formed is then collected in a large tank and sent through pipes to our homes after being purified. The part of the coal that is left consists largely of carbon and is what we call coke. While the gas that comes directly from coal will burn if lighted, it is not a desirable gas to burn in our homes, because it contains a number of substances that should be eliminated before it is used for lighting. How the Gas Is Purified. From the clay retorts the gas passes through horizontal pipes containing water. This cools it and takes out of it most of the tar and water vapor that are driven off with the gas when formed. These substances settle in the water. The gas then goes through a series of curved pipes, which are air cooled. These pipes constitute what is known as an atmospheric condenser. From these the gas goes into a series of receptacles containing wooden slat trays, made up like screens. These receptacles are called the scrubbers, and they take out of the gas the last traces of tar and some of the other compounds found present. The removal of the sulphur is very important, for burning sulphur gives off a gas which is not only extremely impure to breathe, but also injurious to the health. From the scrubbers the gas goes on through pipes to the purifiers--boxes which contain wood shavings coated with iron rust upon which the sulphur is deposited by chemical action. At the same time the lime absorbs a small quantity of carbonic acid gas, which is formed with the other gases. From the purifiers the gas passes into the great iron tanks, in which it is stored until needed. The gas in the tanks consists chiefly of hydrogen, a number of compounds of hydrogen and carbon, and a small amount of a compound of carbon and oxygen containing less oxygen than carbonic acid gas, known as carbon monoxide. The hydrogen and carbon monoxide burn with a very pale flame, which gives but little light and much heat. The light-giving quality of the gas is found in the compounds of carbon and hydrogen. When these burn, the particles of carbon are heated white hot and glow very brightly, making a luminous flame.