The first tests were made as follows:

I had pipes evenly coated with deposits found in house-drain pipes and under the conditions met with in ordinary practice, and made, first, a series of extremely delicate and careful experiments to determine the value of air currents in pipes as a scouring agent.

The maximum rate of this scouring or cleansing was first ascertained by performing the tests under all those conditions which are found to be most favorable to it. Thus the rate of oxidation is greatest when the ventilating current is most rapid, when the temperature is highest and when the largest surface is exposed to the current.

The first experiments were, therefore, performed under these conditions. The waste pipes used were of the diameter of ordinary branch wastes, 1 inches, and were 6 inches long. They were connected with a heated flue by means of an ordinary 1-inch vent pipe in the manner usual in practice, so that the ventilating current should traverse the pipes to be tested from end to end. The time of the year was in midwinter, in January and February.

The pipes were uniformly smeared on the inside with substances found in house drains, using in some common soil from a soil pipe, and in others soap solutions found in lavatory wastes. The deposits were first thoroughly dried in the pipes in order to enable them to be accurately weighed in the laboratory, and they were afterwards moistened three times a day throughout the tests, about as they would be in ordinary practice.

The weight of the deposit in the pipes containing soil was 3.652 grains. That of the lavatory waste was 3.1685 grains. The deposits were then thoroughly moistened with clean water applied with a dropping tube and the pipes connected with the ventilating flue. The velocity of the air current passing through them was then accurately measured and found to be very strong, averaging 8 feet a second, and this velocity was maintained throughout the whole series of tests by means of a stove connected with the main flue, into which the ventilating flue opened. This movement is evidently as rapid as would ever be met with in plumbing practice. The thermometer at the pipe during the tests averaged about 8o° F.

Great care was taken throughout to insure that no foreign substance whatever should get into the pipes tested. No dusting or sweeping was allowed in the rooms and only pure water was used to moisten the deposits. An evaporation of the water used would show a weight of bacteria and other solid matter too small to be detected by our scales. In short, every precaution was taken to obtain reliable results.

After an exposure of a week, under these conditions, to the air current, the pipes were again placed in the air bath for an hour, and the deposits in them thoroughly dried at a temperature of 230° F. Upon weighing, it was found that both deposits had gained in weight. The soil had gained 0.4955 grains and the soapy mixture 0.0130 grains.

Flushing.

The tests were then repeated under the same conditions for a second week. This time the gain of the soil was reduced to 0.4775, and the weight of the lavatory waste was increased to 0.0315 grains.

The bacteria of decomposition and nitrification had evidently not put in their most effective work, and it is probable that the conditions as to light and moisture were not favorable for it. So far the air passed through the pipes was pure air from the room. In the next experiments the ordinary air from the house soil pipe was used and its velocity was 7 feet per second. At the end of a week the soil had lost 0.0575 grains and the lavatory matter 0.0352 grains, which was equivalent in the first case to 1-70th of the entire weight of the deposit and in the second to 1-100th part. Either of these amounts dissolved in water and spread uniformly over the surface of clean pipes of the size of those used was found to be altogether imperceptible to the eye, and the complete purification of these pipes by ventilation under the most favorable circumstances would at this rate require from 70 to 100 weeks, or from 1l/2 to 2 years, supposing there were no addition made to the deposit during the interval through use of the fixture.

We then made the tests with the deposits kept dry as might be the case in pipes under fixtures temporarily in disuse, with approximately the same results, and from all the experiments we reached the conclusion that the solid deposits in the interior of soil and drain pipes are removed so slowly by the oxidation produced by ventilation as to be practically valueless.

The second point investigated, namely, the rate of accumulation in pipes of deposits in ordinary plumbing, required no special experiments. We have ample data in our everyday experience in plumbing. Considering, first, the worst conditions, namely, those of the cold waste pipe from the ordinary kitchen or pantry sink, we know that the accumulation of grease in these will be so rapid as to entirely clog up the pipes in a short time where special precautions are not taken to flush out the pipes from time to time with hot water or some solution of caustic alkali, or where sinks constructed on the principle of an automatic flush tank to be hereafter described,are used. So far as the ventilating current is concerned, however, its well known and generally admitted tendency is to congeal the grease and increase the clogging rather than to diminish it. Consider next the case of an ordinary soil pipe. We find that the tenacious soil will adhere stubbornly to the pipe in masses where it strikes until it is washed away by a powerful fall of water, and that it is not equally distributed in a thin film all over the surface. Parts will be found which are never touched by the waste matter, and parts which are alternately fouled and then scoured clean again. Generally large masses of deposits will be formed in the cavities of the joints or in holes in the castings. In short, the deposits in soil pipes are not slowly distributed favorably for oxidation, but are formed in lumps suddenly, and are either as suddenly removed by the flushing water or are deposited in cavities which largely screen them from the influence of the ventilating current, and therefore in this case also the influence of aeration in removing the solid matter is comparatively very slight. The accumulations of heavy matter will continue in time to increase until they leave an opening only large enough to allow room for the ordinary water flushing stream to pass. Take next the waste pipe from a lavatory. We find the solid deposits here of two kinds, one collecting in clots or masses in corners or unscoured areas, as lumps of soap, hairs, lint, etc., and the other coating the pipes in thin films as of soap-suds. The former are deposited suddenly, and are either swept away by the water or caught in the unscoured cavities and remain there, partially screened from the air current, until other similar substances accumulate above them. The ventilating current, therefore, can have no appreciable effect in removing these masses of matter.

Where, on the other hand, the traps and waste pipes are so constructed and flushed that no such masses can collect, the only kind of deposit that can form in the interval between the flushing will be of the second kind, namely, a thin film of matter like soap-suds, and this the next discharge will remove.

It remains to be seen what effect a powerful water flushing has on these deposits, and this brings us to our third consideration.