Gas Lighting

Gas Lighting is a method of illumination by means of the combustion of coal-gas (q.v.), which is prepared from coal by destructive distillation. Various investigations were made by scientific men concerning the combustible gases that issued from mines in the coal district as far back as 1640. In 1787 Lord Dundonald tried experiments at home on gaslight. In 1813 Westminster Bridge was lit by coal gas, and since then the adoption of this means of illumination has been extensive throughout civilised countries. In most important places in this country the gasworks have now become the property of the local authorities.

The best coal for the purpose is intermediate between anthracite and lignite, of the ordinary bituminous kind. But to prevent the formation of a residue of useless ash it is customary, when employing this kind of coal, to mix it with a certain proportion of harder coal that will give a residue of fairly good coke. The importance of choosing a mixture that gives a good residue is due to the fact that the liquid and solid ingredients which remain after distillation are of considerable value. For example, a ton of good cannel coal distilled between 750° and 800° F. will yield 68 gallons of oil (from which are obtained 2 gallons of paraffin spirit, 22-1/2 gallons of lamp oil, and 24 gallons of heavy oil and paraffin), 1,280 lbs. of coke, while it yields 1,400 cubic feet of gas. If the same be distilled for ordinary gas production in the usual way it yields 18-1/2 gallons of coal-tar (from which are obtained 3 pints of benzol, 3 pints coal-tar naphtha, and 9 gallons of heavy oils) and 1,200 pounds of coke. The distillation must not be prolonged too far if the candle-power is to be a maximum.

The candle-power of the gas is estimated by means of a standard burner burning 5 cubic feet of coal-gas per hour. The burner is either a Sugg's London Argand No. 1, or a standard Steatite Batswing burner. The light is compared with a standard candle (which is a sperm candle 6 to the pound, each burning 120 grains per hour), and it is found that ordinary gas possesses a candle power of 16 to 20 when estimated in the above manner. Better gas can be made, giving a power of 25 to 30 candles.

The general processes for the perfection of the gas when first distilled are given in the article on coal-gas. From the purifier the gas passes to a meter for measuring the quantity of gas manufactured, then through various pressure gauges and recorders, and on through a governor which regulates the flow. Finally it passes into the gas holder, which is an inverted hollow cylinder of wrought-iron held over water that is contained in a cylindrical tank of stone, brick, concrete, or metal. The tank is usually below ground for convenience. As gas passes into the holder the cylinder rises. It, is enabled to rise without difficulty by means of counterweights and vertical guides. The counterweights very nearly balance the weight of the vessel, which therefore only exerts a slight fraction of its weight upon the enclosed gas. Nevertheless the slight pressure of the gas which enters the meter must be sufficient to raise the cylinder. Thus the holders act as reservoirs to supply sufficient gas when the demand is great and store it up when the demand is slight. They are usually made large enough to contain a 21 hours' maximum supply. They also serve to equalise the pressure in the mains. These mains are of cast iron, the joints of the various sections being carefully rendered gas-tight by caulking or by the use of white- and red-lead, indiarubber, or rust cement. Branches pass from the mains to the consumers, each of whom is supplied with a meter to measure the amount consumed.

Gas-meters are of two kinds, dry and wet. The dry meter is an arrangement by which the pressure of gas sends forward a piston or diaphragm first one way along a cylinder and then the other way. The gas on one side of the piston when the cylinder is full is allowed to pass on for consumption, and the piston then returns by pressure of gas on the other side. As each stroke of the piston is made a lever is moved which records the stroke and so measures the quantity of gas passed. The wet meter is rather complicated in construction. It consists of a small inverted cylinder containing 4 spiral chambers. The cylinder is mounted over water on a vertical axis, and as gas passes into one of the spiral chambers the cylinder is slowly rotated. When one chamber is full, the succeeding one is rotated into a convenient position to be filled by gas, and the gas in the first is allowed to pass on by an orifice which discloses itself at the top. The amount of rotation measures the amount of gas supplied, and is recorded on a dial.

Of gas-burners there have been many types invented. It is important that each burner should be governed, as too great a pressure will give a bad flame. The usual burners are of the batswing and fishtail types. The former has a slit across the top of the burner; the latter has two passages meeting at an angle with each other, causing the flame to spread out into a thin sheet. The Argand burner has a hollow ring of flame formed by a number of holes arranged in a circle. The Argand is improved by having a porcelain cylinder in the middle of the flame; it keeps the temperature uniformly high, and also serves to emit light itself when incandescent. The ordinary gas may be improved by being passed through melted naphthaline. It takes up some of this heavy hydro-carbon and is improved in lighting power in consequence. This is the ordinary albo-carbon principle. The average cost of ordinary gas burned in a fairly good burner with a lighting effect of 8 candles is about 1s. per candle per 1,000 hours. Wax candles cost about 34s. per candle per 1,000 hours. Electric arc lamps of 1,500 candle-power cost, on an average, 2-1/2d. per candle per 1,000 hours; while electric incandescent lamps of 16 candle-power cost about 2s. 8d. per candle for the same time.

It is not desirable that the same gas should be employed both for lighting and for heating purposes. It may serve the one function efficiently, but not the other. Nevertheless the evident simplicity of this arrangement, demanding only one set of gas-producers and only one set of supply pipes, is a great argument in favour of the system.