The object of purifying the gas is to remove from it two deleterious components, namely, sulphuretted hydrogen and carbonic acid. The first compound should be completely removed, owing to its poisonous and destructive properties, its presence being shown very rapidly by the gray tinge which it gives to all lead paint, and the brown tinge it produces on all gilding, plating, and silver work. The presence of carbonic acid in the gas renders the illumination less brilliant. The appearance of both impurities can be rapidly and easily ascertained, and should ensure the immediate discharge of the purifying material from a "box". The means of turning the stream of gas from one purifier to another is invariably provided in gas-works. It is known as a "by-pass", and by its use the gas is passed through a clean purifier, while the other, which has become 'foul", is being changed. Soft gray lime proves the best purifying agent, and should not be slaked till within twenty-four hours of using.

The purified gas is led to the meter, and from thence to the gasholder. The meter is an instrument designed for the measurement of the volume of gas made, but it is not a necessary adjunct to a gas-works, as a gasholder will act as an approximate "tell tale" of the quantity of gas made and consumed.

A gasholder for the capacity of works under our notice would have an out-side diameter of 12 to 16 feet. The structure consists (as shown in Plate XXIV.) of an outer tank about 8 feet deep, made of wrought-iron sheeting, and having a ring of angle iron, or member of some similar section, around the top edge. Attached to this tank are the columns and guides, which rise in a vertical position rather more than 8 feet above the rail of the tank. Inside this tank is placed an inverted tank of slightly-diminished diameter, which is restrained from any side or circular movement by roller guides fixed on the top and bottom, which engage with guides connected to the inside of the columns. The only movements allowed to this "holder" are consequently those of rising and falling. When filled with gas the holder rises, owing to the effect of the pressure exerted on the top or "crown". The same pressure is felt upon the water placed in the tank to form a "seal", and is counteracted by lifting a quantity of the liquid outside in the annular space between the holder and the inside of the tank, which is only exposed to the pressure of the atmosphere. The difference in height between the outside and inside levels of water in the gasholder-tank gives what is technically known as the "gas-pressure" at that point; in other words, gas-pressure is the height of a column of water in air maintained by the pressure of gas. It is an invariable object to reduce the pressure of gasholders; the wrought-iron sheeting of the holder is therefore of very light material. The inlet and outlet gas-pipes to the holder are carried up through the bottom plate of the tank above the level of the water.

A "governor" is placed on the outlet-pipe from the gasholder, for the purpose of modifying any variations in the pressure of the gas due to the rising and falling of the holder, or depressions given to it by gusts of wind. Such differences in pressure are instantly felt where this instrument is not used, causing the flames in burners to "jump" or vary in size rapidly, instead of burning with a steady Maine as is the case under a constant pressure of gas. The position of the "governor" is frequently close to the point of consumption. This apparatus1 is of small bulk, and can be fitted inside the house without difficulty or detriment, and is so designed that, with either five or fifty burners in action, the pressure of gas supplied to them remains constant.

Coal-gas is without doubt the most amenable and well-conditioned gas for purposes of illumination. It is a mixture of compounds of hydro-carbons, other carbon bodies, and hydrogen, very stable in composition, and of light density. It is an excellent heat-agent, and easily distributed. It possesses one unfortunate characteristic, viz. that of depositing naphthalene crystals in the outdoor service-pipes, but this never becomes a source of trouble in small works, and rarely occurs when cannel-coal or shale is used Preference should be given, however, to Durham coal or Yorkshire Silkstone, on the score of cheapness, and freedom from benzine constituents in the composition of the gas. Further, where the illumination is effected by means of the excellent ventilating regenerative burners, - such as the Wenham2 and Vertmarche, - the illuminating power of a cubic foot of cannel gas is very slightly above that of gas produced from common coal.

1 See figs. 643-646, pp. 286-287, VoL II. - Ed.

2 See figs. 651-653, pp. 291-292, VoL II.-ED