The main waste-pipes, in dwelling-houses, are almost always of cast iron, put together with "caulked joints" of melted lead, wrought-iron pipe, with joints screwed together, being used only for public buildings, and for the most expensive class of dwellings; and the main air-pipes, which are closely connected with the waste-pipes, are usually of the same material. The smaller waste and air pipes, and the supply-pipes, may be of lead, iron, or brass, or of a mixture of these materials. As waste and air pipes do not have to endure great pressure, the smaller ones are usually of lead, which has the advantage of being easily joined, and being practically exempt from corrosion; but supply-pipes are often subjected to great pressure, which only a very thick lead pipe can resist, and some waters, particularly rain-water, or very pure river or spring water, dissolve lead in sufficient quantity to make them slightly poisonous. Where this is the case, the best material for supply-pipes, although an expensive one, is brass, Iron supply-pipes, which are much cheaper, sooner or later fill up with rust. Galvanizing or enamelling will postpone the rusting, often for a considerable period, but galvanized pipes frequently impart an injurious contamination to the water that runs through them. The tin-lined and lead-lined pipes, which are now much used, are good if put in with care, but, if handled by inferior plumbers, who cut them without proper caution, the iron may be exposed at a joint; and the pipe may fill with rust from a small surface of iron left bare in this way, as the galvanic action between the metal of the lining and the iron pipe assists greatly the corrosion of the latter.

Pipes.

Brass pipes, on the contrary, are not subject to corrosion by any ordinary water. They are, however, unless annealed, liable to internal strains, which often cause them to split without warning, and they should be of "iron-pipe sizes," the light pipes known under the name of "plumbers' tubing " being too thin to make substantial joints; but, if of proper weight and quality, and intelligently put together, and kept from freezing, brass supply-pipes are practically everlasting.

It is usual in "open plumbing " to make all the traps and pipes, including supply, waste, and air pipes, which are exposed in bath-rooms, of brass, nickel-plated, for the sake of the bright and clean appearance which they present; but nickel-plating, where it cannot be easily reached for cleaning, soon becomes covered with oxide, of a dingy, greenish gray, and it is becoming common, instead of plating such pipes, to bronze them with silver or aluminum bronze, which can be applied to lead and iron pipes, as well as to those of brass, and is easily renewed, if necessary.

Tin-lined pipes.

Brass pipes.

Nickel-plating.

Even where the cold-water supply-pipes are of lead, it is common to make the hot-water pipes of brass, for the reason that lead hot-water pipes, through alternate expansion and contraction, as the hot water is drawn through them, gradually acquire a series of bends between their supports, which are unsightly, and check the flow of water through them; so that the strong and rigid brass pipes are more satisfactory. Brass hot-water pipes must, however, be put up with angles in their course, so that the expansion and contraction may be taken up by the spring of the angle, as a long, straight pipe, held firmly at the ends, will soon leak at the joints, through the strain which is brought upon them by contraction and expansion.

As hot-water pipes often give trouble in other ways besides leaking at the joints, it is desirable to understand the principles upon which they are, or should be, put in. The manner in which the water in the bath-boiler is heated from the water-front in the kitchen stove has already been described in treating of the latter. The bath-boiler itself is a copper or galvanized-iron cylinder, mounted on a stand, with a coupling in the side, to receive the flow-pipe from the water-front of the range, another in or near the bottom, for the return-pipe to the range, and three or more couplings in the top. The middle coupling on top connects with a tube which extends down, inside the boiler, nearly to the bottom. This is used for the supply of fresh cold water which enters the boiler to replace the hot water that is drawn off; and the tube inside the boiler serves to conduct it to the bottom of the boiler without allowing it to mix with the hot water which occupies the upper portion of the latter. The other couplings in the top of the boiler are for the pipes which carry the hot water over the house. The action of the boiler, as thus fitted up, is very simple. So long as no hot water is drawn from it, the water that it contains simply circulates through the water-front of the range and back again, getting gradually hotter in the process; but, as soon as any hot water is drawn from the boiler, by means of the pipes which extend from its top, an equivalent amount of cold water descends through the inside tube to the bottom of the boiler, and thence to the waterfront, where it takes its place in the general circulation.

Hot-water pipes.

Hot-water piping system.

Bath-boilers.

Strictly speaking, this constitutes all the necessary part of a hot-water system, including a provision for supplying fresh water as hot water is drawn off; a water-front for heating the water; a boiler, or more correctly, a reservoir, for storing a reserve of water heated in the range, and a system of pipes to distribute the water from the boiler; but, if no circulation is provided except that between the water-front and the boiler, the pipes above the boiler will be cold; and, on opening a hot-water faucet anywhere, a certain amount of cold water, depending upon the length of the pipe, must be drawn before any hot water reaches the faucet.

To obviate this annoyance, it is usual to provide, where circumstances permit, one or more secondary circulations, by which the main hot-water pipes, after reaching the highest fixture that they supply, are carried over in a loop, and brought down again to the boiler, entering the latter near the bottom. In this way the water in the loop is constantly rising hot from the boiler, passing through the circulation-pipe, and back again, only slightly cooled, to the boiler; so that hot water can be drawn immediately from any faucet on the line. This arrangement is, however, only practicable where the hot-water system is supplied from a tank in the upper story, for the reason that bubbles of air collect in an upward bend of any pipe conveying water, and will stop the circulation unless they are allowed to escape; so that, where a circulation-pipe is brought back to the boiler from the highest part of a hot-water system it is necessary, in order to preserve the flow through it, to carry up from the highest point an expansion-pipe, which is extended above the tank