In the average-sized dwelling where non-siphoning traps are used, back-venting is not necessary, but in large houses and in plumbing where siphon traps are used, vent pipes must be attached to the traps to assure a satisfactory system. Fig. 100 furnishes an example of back-venting, applied to the bathroom shown in Fig. 99. In the former figure the bath tub and wash basin are connected with the waste pipe by siphon traps. A siphon trap may lose its seal in two ways: by self-siphonage, or by aspiration caused by the discharge of the water from other fixtures. In the discharge of the siphon trap, such as _B_, in Fig. 100, the long leg of the siphon, formed by the discharge pipe, may carry away the water so completely that not enough remains in the trap to form a seal. Again, the discharge of the water from the bath tub through the waste pipe tends to form a vacuum above it and in some cases the seal in _B_ is destroyed by the water being drawn into the vertical pipe. The possibility of either of these occurrences is prevented by back-venting. [Illustration: FIG. 100.--An example of back-vented plumbing as applied to the bathroom.] In Fig. 100, a pipe from the main stack is connected with the bend of the trap at _B_ and also to the waste pipe outside the trap at _T_. A vent is also taken from the drain _C_, at a point just below the trap in the closet seat. The object of all of the vents is to prevent the tendency of the formation of a vacuum from any cause that will carry away the water seal of the trap and allow sewer gas to enter the house. The closet seat also contains a trap which will be described later. It connects with soil pipe _S_, leading to the sewer by a large lead pipe _C_. All of the pipes under the floor, leading to the soil pipe, should be of lead. The pipes above the floor are generally of iron or nickel-plated brass. All of the connections in the lead pipes are made with _wiped joints_; that is, the connections are made by wiping hot solder about the joint, in a manner peculiar to this kind of work, in such a way as to solder the pipes together. The joints made in this manner are perfectly and permanently tight. Lead pipes are used under such conditions, because lead is the least affected by corrosion of any of the metals that could be used for such work.