Whenever still better light is required in railway carriages, as also for the illumination of large, constantly used vehicles, such as omnibuses, the acetone process (cf. Chapter XI.) exhibits notable advantages. The light so obtained is the light of neat acetylene, but the gas is acetylene having an upper limit of explosibility much lower than usual because of the vapour of acetone in it. In all other respects the presence of the acetone will be unnoticeable, for it is a fairly pure organic chemical body, which burns in the flame completely to carbon dioxide and water, exactly as acetylene itself does. If the acetylene is merely compressed into porous matter without acetone, the gas burnt is acetylene simply; but per unit of volume or weight the cylinders will not be capable of developing so much light.
In the United States, at least one railway system (The Great Northern) has a number of its passenger coaches lighted by means of plain acetylene carried in a state of compression in cylinders without porous matter. The gas is generated, filtered from dust, and stored in an ordinary rising holder at a factory alongside the line; being drawn from this holder through a drier to extract moisture, and through a safety device, by a pump which, in three stages, compresses the acetylene into large storage reservoirs. The safety device consists of a heavy steel cylinder filled with some porous substance which, like the similar material of the acetone cylinders, prevents any danger of the acetylene contained in the water-sealed holder being implicated in an explosion starting backwards from the compression, by extinguishing any spark which might be produced there. The plant on the trains comprises a suitable number of cylinders, filled by contact with the large stores of gas to a pressure of 10 atmospheres, pipes of fusible metal communicating with the lamps, and ordinary half-foot acetylene burners. The cylinders are provided with fusible plugs, so that, in the event of a fire, they and the service- pipes would melt, allowing the gas to escape freely and burn in the air, instead of exploding or dissociating explosively within the cylinders should the latter be heated by any burning woodwork or the like. It is stated that this plan of using acetylene enables a quantity of gas to be carried under each coach which is sufficient for a run of from 53 to 70 hours' duration, or of over 3600 miles; that is to say, enables the train, in the conditions obtaining on the line in question, to make a complete "round trip" without exhaustion of its store of artificial light. The system has been in operation for some years, and appears to have been so carefully managed that no accident has arisen; but it is clear that elements of danger are present which are eliminated when the cylinders are loaded with porous matter and acetone. The use of a similar system of compressed acetylene train lighting in South America has been attended with a disastrous explosion, involving loss of life.
It may safely be said that the acetone system, or less conveniently perhaps the mere compression into porous matter, is the best to adopt for the table-lamp which is to be used in occupied rooms Small cylinders of such shapes as to form an elegant base for a table-lamp on more or less conventional lines would be easy to make. They would be perfectly safe to handle. If accidentally or wilfully upset, no harm would arise. By deliberate ill-treatment they might be burst, or the gas-pipe fractured below the reducing valve, so that gas would escape under pressure for a time; but short of this they would be as devoid of extra clangor in times of fire as the candle or the coal-gas burner. Moreover, they would only contaminate the air with carbon dioxide and water vapour, for the gas is purified before compression; and modern investigations have conclusively demonstrated that the ill effects produced in the air of an imperfectly ventilated room by the extravagant consumption of coal-gas depend on the accumulation of the combustion products of the sulphur in the gas rather than upon the carbon dioxide set free.
One particular application of the portable acetylene apparatus is of special interest. As calcium carbide evolves an inflammable gas when it merely comes into contact with water, it becomes possible to throw into the sea or river, by hand or by ejection from a mortar, a species of bomb or portable generator which is capable of emitting a powerful beam of light if only facilities are present for inflaming the acetylene generated; and it is quite easy so to arrange the interior of such apparatus that they can be kept ready for instant use for long periods of time without sensible deterioration, and that they can be recharged after employment. Three methods of firing the gas have been proposed. In one the shock or contact with the water brings a small electric battery into play which produces a spark between two terminals projecting across the burner orifice; in the second, a cap at the head of the generator contains a small quantity of metallic potassium, which decomposes water with such energy that the hydrogen liberated catches fire; and in the third a similar cap is filled with the necessary quantity of calcium phosphide, or the "carbophosphide of calcium" mentioned in Chapter XI., which yields a flame by the immediate ignition of the liquid phosphine produced on the attack of water. During the two or three seconds consumed in the production of the spark or pilot flame, the water is penetrating the main charge of calcium carbide in the interior of the apparatus, until the whole is ready to give a bright light for a time limited only by the capacity of the generator. It is obvious that such apparatus may be of much service at sea: they may be thrown overboard to illuminate separate lifebuoys in case of accident, or be attached to the lifebuoys they are required to illuminate, or be used as lifebuoys themselves if fitted with suitable chains or ropes; they may be shot ahead to illuminate a difficult channel, or to render an enemy visible in time of war. Several such apparatus have already been constructed and severely tested; they appear to give every satisfaction. They are, of course, so weighted that the burner floats vertically, while buoyancy is obtained partly by the gas evolved, and partly by a hollow portion of the structure containing air. Cartridges of carbide and caps yielding a self- inflammable gas can be carried on board ship, by means of which the torches or lifebuoys may be renewed after service in a few minutes' time.
[CHAPTER XIV]
VALUATION AND ANALYSIS OF CARBIDE
The sale and purchase of calcium carbide in this country will, under existing conditions, usually be conducted in conformity with the set of regulations issued by the British Acetylene Association, of which a copy, revised to date, is given below:
"REGULATIONS AS TO CARBIDE OF CALCIUM."
1. The carbide shall be guaranteed by the seller to yield, when broken to standard size, i.e., in lumps varying from 1 to 2-1/2 inches or larger, not less than 4.8 cubic feet per lb., at a barometric pressure of 30 inches and temperature of 60° Fahr. (15.55° Centigrade). The actual gas yield shall be deemed to be the gas yield ascertained by the analyst, plus 5 per cent.
"Carbide yielding less than 4.8 cubic feet in the sizes given above shall be paid for in proportion to the gas yield, i.e., the price to be paid shall bear the same relation to the contract price as the gas yield bears to 4.8 cubic feet per lb.