BESIDES the well known enemies of the water seal of traps already described, another exists which is, however, more insidious and no less fatal in its action. It works like the vampire, silently and stealthily, drawing the life from the trap, without warning of any kind, and often leaves open the gates of the sewers without detection. Where the trap is constructed entirely of opaque material the absence of the water seal cannot be seen, and where glass is used it soon becomes coated with an opaque film, so that the source of the leakage of sewer air into the house cannot be seen and, as capillary-action is unsuspected, the presence of sewer air is attributed to some other cause.

Capillary action is the subtle thief which does this mischief. Hairs, lint, bits of twine, paper, sponge or matted fibrous filth of a great variety of kinds are the tools with which it operates. A small quantity of any of these substances, forming a continuous mass or chain from the water in the trap to and over into the outflow, will, under certain conditions, soak up and slowly drain off the water from the trap until the seal is destroyed. Let us examine the conditions favorable to this action and ascertain by what means, if any, it may be prevented.

In books on the subject of capillarity we find the theory explained with scientific accuracy. The precise amount of

Fig. 283. Capillary

Fig. 283. Capillary elevation of liquids in tubes of very fine bore, nominally of the diameter of a hair (capilla), is calculated in these treatises to a nicety, and we are in some of them referred for complete satisfaction and elucidation particularly to the gigantic work of La Place on "Celestial Mechanics" (tenth book, supplement). Knowing from the study of this interesting work that the tension of the surface of contact of two liquids or bodies is represented by the equation Capillary Action 292 what more need the practical plumber have to cause the whole subject of the capillary effect of sediment in traps and the best methods of dealing with it to burst upon his delighted understanding in a flood of light ? All he requires is a knowledge of the higher mathematics and some skill and ingenuity in arranging his data for calculation. He knows from the treatises that the finer the bore of the tube the higher the liquid will rise in it, provided the surfaces are of a kind the liquid can wet; that plane surfaces which can be wetted by a liquid will exert a similar attraction on liquids, provided they are put near enough together, not exceeding 1-10 of an inch apart, and that the attractive power is in proportion to the proximity of the surfaces and independent of the thickness of the bodies underlying them. But he will not find in the treatises what the exact effect will be on liquids of the interposition of numerous plane and rounded surfaces such as are presented by the sediment found in traps, under the peculiar conditions of surroundings, temperature and moisture met with in plumbing. Inasmuch as these peculiar conditions would render his calculations somewhat more complicated and difficult, and as the books have not investigated the subject sufficiently, a study of this particular branch of the subject from a practical rather than a theoretical standpoint seems needed.