It is evident that an habitual breathing of air in which even minute doses of toxic substances and gases are floating will in time impair the health of human beings, and that large doses of those substances may be directly toxic and dangerous to health. It is certainly an error to ascribe to sewer air death-dealing properties, but it would be a more serious mistake to undervalue the evil influence of bad sewer air upon health. =Ventilation.=--To guard against the bad effects of sewer air, it is necessary to dilute, change, and ventilate the air in sewers. This is accomplished by the various openings left in the sewers, the so-called lamp and manholes which ventilate by diluting the sewer air with the street air. In some places, chemical methods of disinfecting the contents of sewers have been undertaken with a view to killing the disease germs and deodorizing the sewage. In the separate system of sewage disposal, where sewer pipes are small and usually self-cleansing, the late Colonel Waring proposed to ventilate the sewers through the house pipes, omitting the usual disconnection of the house sewer from the house pipes. But in the combined system such a procedure would be dangerous, as the sewer air would be apt to enter the house. Rain storms are the usual means by which a thorough flushing of the street sewers is effected. There are, however, many devices proposed for flushing sewers; e. g., by special flushing tanks, which either automatically or otherwise discharge a large volume of water, thereby flushing the contents of the street sewers. CHAPTER VI =Plumbing= =Purpose and Requisites for House Plumbing.=--A system of house plumbing presupposes the existence of a street sewer, and a water-supply distribution within the house. While the former is not absolutely essential, as a house may have a system of plumbing without there being a sewer in the street, still in the water-carriage system of disposal of sewage the street sewer is the outlet for the various waste and excrementitious matter of the house. The house-water distribution serves for the purpose of flushing and cleaning the various pipes in the house plumbing. The purposes of house plumbing are: (1) to get rid of all excreta and waste water; (2) to prevent any foreign matter and gases in the sewer from entering the house through the pipes; and (3) to dilute the air in the pipes so as to make all deleterious gases therein innocuous. To accomplish these results, house plumbing demands the following requisites: (1) _Receptacles_ for collecting the waste and excreta. These receptacles, or plumbing fixtures, must be adequate for the purpose, small, noncorrosive, self-cleansing, well flushed, accessible, and so constructed as to easily dispose of their contents. (2) _Separate Vertical Pipes_ for sewage proper, for waste water, and for rain water; upright, direct, straight, noncorrosive, water- and gas-tight, well flushed, and ventilated.