The agent used is either lime or hydrated oxide of iron, and by their means the gas is robbed of its carbonic acid and the greater part of its sulphur compounds. The process is then considered complete, and the gas passes on into the water chamber over which the gas-holder is reared, and in which it rises through the water, forcing the huge cylinder upward according to the pressure it exerts.
The gas-holder is poised between a number of upright pillars by a series of chains and pulleys, which allow of its easy ascent or descent according as the supply is greater or less than that drawn from it by the gas mains.
[Illustration: FIG. 38.]
When we see the process which is necessary in order to obtain pure gas, we begin to appreciate to what an extent the atmosphere is fouled when many of the products of distillation, which, as far as the production of gas is concerned, may be called impurities, are allowed to escape free without let or hindrance. In these days of strict sanitary inspection it seems strange that the air in the neighbourhood of gas-works is still allowed to become contaminated by the escape of impure compounds from the various portions of the gas-making apparatus. Go where one may, the presence of these compounds is at once apparent to the nostrils within a none too limited area around them, and yet their deleterious effects can be almost reduced to a minimum by the use of proper purifying agents, and by a scientific oversight of the whole apparatus. It certainly behoves all sanitary authorities to look well after any gas-works situated within their districts.
Now let us see what these first five products of distillation actually are.
Firstly, house-gas. Everybody knows what house-gas is. It cannot, however, be stated to be any one gas in particular, since it is a mechanical mixture of at least three different gases, and often contains small quantities of others.
A very large proportion consists of what is known as marsh-gas, or light carburetted hydrogen. This occurs occluded or locked up in the pores of the coal, and often oozes out into the galleries of coal-mines, where it is known as firedamp (German dampf, vapour). It is disengaged wherever vegetable matter has fallen and has become decayed. If it were thence collected, together with an admixture of ten times its volume of air, a miniature coal-mine explosion could be produced by the introduction of a match into the mixture. Alone, however, it burns with a feebly luminous flame, although to its presence our house-gas owes a great portion of its heating power. Marsh-gas is the first of the series of hydro-carbons known chemically as the paraffins, and is an extremely light substance, being little more than half the weight of an equal bulk of air. It is composed of four atoms of hydrogen to one of carbon (CH_{4}).
Marsh-gas, together with hydrogen and the monoxide of carbon, the last of which burns with the dull blue flame often seen at the surface of fires, particularly coke and charcoal fires, form about 87 per cent. of the whole volume of house-gas, and are none of them anything but poor illuminants.
The illuminating power of house-gas depends on the presence therein of olefiant gas (ethylene), or, as it is sometimes termed, heavy carburetted hydrogen. This is the first of the series of hydro-carbons known as the olefines, and is composed of two atoms of carbon to every four atoms of hydrogen (C_{2}H_{4}). Others of the olefines are present in minute quantities. These assist in increasing the illuminosity, which is sometimes greatly enhanced, too, by the presence of a small quantity of benzene vapour. These illuminants, however, constitute but about 6 per cent. of the whole.
Added to these, there are four other usual constituents which in no way increase the value of gas, but which rather detract from it. They are consequently as far as possible removed as impurities in the process of gas-making. These are nitrogen, carbonic acid gas, and the destructive sulphur compounds, sulphuretted hydrogen and carbon bisulphide vapour. It is to the last two to which are to be attributed the injurious effects which the burning of gas has upon pictures, books, and also the tarnishing which metal fittings suffer where gas is burnt, since they give rise to the formation of oil of vitriol (sulphuric acid), which is being incessantly poured into the air. Of course the amount so given off is little as compared with that which escapes from a coal fire, but, fortunately for the inmates of the room, in this case the greater quantity goes up the chimney; this, however, is but a method of postponing the evil day, until the atmosphere becomes so laden with impurities that what proceeds at first up the chimney will finally again make its way back through the doors and windows. A recent official report tells us that, in the town, of St Helen's alone, sufficient sulphur escapes annually into the atmosphere to finally produce 110,580 tons of sulphuric acid, and a computation has been made that every square mile of land in London is deluged annually with 180 tons of the same vegetation-denuding acid. It is a matter for wonder that any green thing continues to exist in such places at all.