The _smoke-test_ consists in filling the drains with smoke, the assumption being that this will find its way through any faulty joint or trap, thus indicating the site of the defect. Various arrangements are employed for pumping the smoke into the drain from the combustion chamber of a pumping apparatus; or smoke is produced by means of specially prepared rockets. All outlets or ventilating pipes must be carefully stopped during the operation, and the place where the smoke is smelt will then indicate any leaky point. [Illustration: FIG. 35. SHOWING STOPPER FOR WATER-TESTING OF DRAINS.] Drain grenades are largely employed for the volatile testing of drains, the essential constituent being phosphide of calcium. The grenade, which is attached to a piece of string, is passed beyond the trap of the w.c., and as the string unwinds the grenade opens and discharges its contents into the soil-pipe. Or a tablespoonful of strong oil of peppermint, mixed with hot water, is poured down the highest water-closet in the house. If this is smelt by another person in the lower closets, it indicates defective traps or soil-pipe. All volatile and smoke tests have but a limited utility. They are useful in detecting defective joints in traps and in the soil-pipe. They may detect defects in an underground drain; but if no smell or smoke is perceptible when a drain is tested by this means, the drain may still be seriously defective. The only absolutely trustworthy test for drains is the =hydraulic or water test=. The lower end of the drain is stopped up by a suitable water-tight stopper. Then the drain is filled with water by means of a tap in the yard, the amount of water used being approximately estimated by the rate of flow from this tap. The drain is filled up to the level of the gully-traps in the yard. If it remains at this level for half an hour, the drain is sound. More often it leaks so rapidly that it will not fill, or the level of the water falls quickly after filling, and it is necessary to strip and repair, or more generally to relay the drain so as to make it water-tight. Rats are an important indication of defective drains. The presence of rats in a house should always lead to a thorough investigation of its drains. CHAPTER XXVI. CESSPOOLS AND MAIN SEWERS. The terms =Sewer and Drain= are used somewhat confusedly. The term _drain_ should be used to designate the pipes bringing the sewage from the house into the street-sewer, or any pipes by which the subsoil is drained; the term _sewer_ being confined to the trunk canals into which the house drains empty their contents. Where the water-carriage system of sewerage is adopted, involving the use of water-closets as described in the last chapter, the sewage may be carried from the house either into cesspools or into the main sewer.