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.