Fig. 374.—Experiment showing that carbonic acid contains oxygen and carbon.
The compounds of carbon with hydrogen are important. There is the “light” carburetted hydrogen (CH4), which is usually known as fire-damp in coal mines. It is highly inflammable and dangerous. The safety-lamp invented by Davy is a great protection against it, for as the gas enters it is cooled by the wire, and burns within harmlessly. The explosion warns the miner. “Heavy” carburetted hydrogen possesses double the quantity of carbon (C2H4). It is also explosive when mixed with oxygen.
Fig. 375.—Temperature reduced by contact with wire.
The most useful compound is coal-gas, and though its principal function appears to be in some manner superseded by electricity, “gas” is still too important to be put aside. It can easily be obtained by putting small fragments of coal in the bowl of a tobacco-pipe, closing the bowl with clay, and putting it in the fire. Before long the gas will issue from the stem of the pipe, and may either be lighted or collected in a bladder. For the use of the “million,” however, gas is prepared upon a very large scale, and is divided into three processes—its “formation,” “purification,” and its “collection” for distribution to consumers. The first process is carried on by means of retorts shown in the illustration (fig. 376). The first portion of the next figure is a section of a furnace, the other part shows two furnaces from the front. The following is the mode employed. The coal is put into retorts fitted to the furnace, so that they are surrounded by the flames, and terminating in a horizontal tube called the hydraulic main, E, which is in its turn connected with a pit or opening for the reception of the tar and ammoniacal liquor, etc., which condenses from the gas. It then passes up and down a series of tubes in water, called a “condenser,” and in this are reservoirs or receptacles for any tar and ammonia that remain. But sulphur is still present, so the gas is carried to the purifying apparatus (D in fig. 378), which consists of a large cylindrical vessel air-tight, with an inverted funnel, nearly filled with a mixture of lime and water. The gas bubbles in, and the sulphur unites with the lime, while the gas rises to the top (trays of lime are used when the gas enters from the bottom). The Gasometer, a large vessel closed at the top and open below, dips into a large trough of circular shape. The gasometer is balanced by weights and chains, and may be raised (See fig. 379). When quite empty the top rests upon the ground, and when the gas enters it is raised to the top of the frame which supports it. We have now our Gasometer full. When the time comes to fill the pipes for lighting purposes, some of the weights are removed, the Gasometer falls down slowly, and forces the gas through the tubes into the main supply to be distributed. About four cubic feet of gas is obtained from every pound of coal. When gas and air become mixed, the mixture is very explosive. In a house where an escape of gas is detected let the windows be opened at the top, and no light introduced for several minutes.
Fig. 376.—Retorts.
Fig. 377.—Section. Front view.
