Lead pipe is the mast suitable for conveying gas in houses. It can be readily bent and fixed in all positions. The joinings can be soundly made, even when numerous connections come close together, and alterations or additions are easily effected. Lead pipes do not corrode, and no dust is given off to choke up the fittings or burners. There are, however, several disadvantages in the use of lead gas-pipes, for, if badly laid, they sag, forming loops or festoons between the fastenings, and these loops become water-logged owing to the condensation of vapour, and cause the lights to jump and ultimately go out In houses subject to vibration lead pipes break, usually about the staircases, and they are also liable to be pierced with nails when hanging pictures or ornaments.

Lead gas-pipes are made from inferior lead, and when buried in the plaster the lime frequently affects the lead to such an extent as to cause it to crack every few inches.

Composition pipes are not so much used now as formerly. They are harder and stiffer than lead, and not so liable to be affected by lime. They require a little more skill to join neatly, but the joints are equally as good and sound as the joints on lead pipes.

Iron pipes are most suitable for workshops, and all other places where the pipes are exposed, or have to be suspended. They are sometimes used in houses, and give no further trouble after being once properly fixed, except from rust and dirt

The service-pipe from the street-main to about 6 inches inside the house is usually of iron with ordinary screwed joints. Galvanized-iron pipes are used in some places, and lead in others. Both lead and plain iron service-pipes should be surrounded with a loose wood casing, which, in the case of the iron, is afterwards filled with a composition of pitch.

Outside the premises a stop-cock is fixed for the use of the company's men, and another is always fixed as soon as the main enters the building. The connection from this stop-cock to the meter is usually of lead, the same size as the vice-pipe and tap, and is soldered to the unions provided by means of a copper-bit or spirit blow-pipe, using resin as a flux.

The outlet-pipe from the meter will be the same size as the inlet, and will be continued - in the case of houses - past the centre of the first floor, when it can be reduced to suit the number of lights supplied by the branches. The supply-pipe to the second floor will also l)e of less bore, and the same again applies to the third floor. With a 10-light or 15-light meter, the supply to the first floor will be 1 inch, to the second J inch or inch, and to the third inch or J inch. Where there are four lights to be supplied on the first and second floors, a -inch braneh should be run past the first two, and afterwards continued with -inch pipe. The same four lights on the third floor would only require 3/8-inch pipes throughout, owing to the pressure increasing the higher the pipes are carried. In good work, -inch pipes are seldom used on account of their weakness, leakages being frequent, and because a very small drop of water is sufficient to cause the lights to jump. If these pipes are used, they should never bc fixed horizontally on this account, and in some places they are not allowed at all.

In fixing the gas-pipes care must be taken to have a continuous fall back to the meter, in order to prevent the lodgment of the condensed vapour in any part of the pipes. If sufficient care is taken in arranging the runs, there will be no necessity to fix syphons; indeed syphons are usually a sign of unskilled labour. They were at one time pretty numerous, but are now rarely met with, except where alterations or additions have been made.

When the main service-pipes are laid in private ground between the house and the street-main, and the distance is too great to allow of a fall to the street-main, one or more syphon-boxes will be necessary. All such boxes are provided with taps to drain off the water collected in them. When the gas-mains are continued outside the house to supply the out-buildings, syphons will also be required according to the position and length of the pipes. A short length of iron pipe with the end capped up, or a piece of lead pipe with a tap at the end, is often used instead of a syphon-box, but requires to be emptied oftener and more regularly than the latter.

Short lengths of pipe having blank ends are termed syphons (the boxes used as receivers being termed syphon -boxes), not because they are syphons, but because their predecessors were inverted syphons, - that is, a pipe bent in the form of the letter U, - the water forming a trap and preventing the escape of gas, so long as the water draining from the pipes equalled or exceeded that evaporating from the exposed end of the syphon. In many cases the quantity evaporated exceeded the supply, with the result that serious explosions occurred, and the use of the syphon was prohibited; but the name still remains, and although syphon-pipes and syphon-boxes are terms often met with, it must be understood that they have no connection with the syphon proper, as they are merely pipes or boxes fixed to receive and retain the condensed water.

In the case of wet meters up to 50 lights, the pipes must be laid to fall to the meter, the condensed water helping to keep the meter charged. Where a dry meter is fixed, the water must on no account be allowed to drain into it. The main supply-pipe in the building should be carried down below the meter, and the outlet-pipe from the meter branched into it in such a way as to stop the gas before the water can overflow into the meter when the syphon is full; this can be done, as shown in fig 642, by bending down the end of the outlet-pipe so as to bring the branch-joint below the underside of the bent pipe, thus forming a trap, which will indicate by the fluctuations of the gas-lights that the syphon is fall. If connected as shown by the dotted lines, the water can drain into the meter.