This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Although it is customary now to run the plumbing pipes outside of the plaster, there will be some ventilation pipes that will need to be provided for at this time. All of the enclosed gas piping must be done before lathing. From the fact that the gas piping must be carried in almost every case to the center of the ceilings, there is need of careful watching lest the cutting by careless workmen shall weaken the joists. The specifications distinctly state that no floor beams shall be cut into, more than two feet away from their bearing, but as this means that all outlets in the center of ceilings must be reached by branch pipes between the joists, entailing more piping and labor for the gas fitter, there is a great temptation to disregard instructions, and to cut the joists for a straight run from center to center of rooms. Only constant watching will prevent this being done. Care must be taken that the pipes are run with a continuous drop toward the meter, to allow the liquid, which will always be condensed from the gas, to run off; and for the same reason, wall outlets must be piped up from below and ceiling drops should be taken out of the side or top of the pipes. The position of all outlets must be verified, especially those which are centered in the rooms, and they must be set at right angles to walls and ceilings. This can be verified by screwing on to each outlet, as soon as set, a piece of pipe a foot or more in length and testing with a steel square, No "gas fitters" or other cement should be allowed, but all pipes should be put together with red lead. The outlets to receive fixtures should be strongly secured to prevent springing or movement in the plastered work.
"When the piping is all in place the outlets are to be capped and the whole system tested for leaks, and accepted by the local gas company. This test consists, briefly, in attaching to one of the outlets a mercury gauge, and then filling the pipes with air under pressure till the mercury in the gauge stands to the required height to insure tightness, which is usually six to twelve inches. The apparatus is then left to stand for ten or fifteen minutes, and if the mercury still holds the same level it is safe to say that the pipes are tight. Leaks may usually be detected by the sound of the escaping air, but often ether is put into a cup attached to the pump and forced in with the air, so that leaks, especially in concealed parts of the piping, may be detected by the odor of the ether. Suspicion of a leak in pipe or fittings may be verified by brushing strong soap-water over the place, when a bubble will be blown by the escaping air. Small pin holes in the pipe or couplings may be tamped, and if thus rendered tight will remain so, but pipes showing a split or large hole must be replaced.