Dr. Carmichael's experiments were made to ascertain to what extent the contents of the soil pipe, both gases and germs, are able to pass through a sound water trap.

Dr. Andrew Fergus had shown that concentrated gases would pass through a water trap, and had had glass tubes bent in the form of a trap to show this. Ammonia was detected on the other side of the trap in 15 minutes, sulphuretted hydrogen in from 3 to 4 hours, and carbonic acid in1 l/2 hours. But the gases were presented to the under surface of the water in a very concentrated condition, probably not under 50 per cent. Dr. Carmichael wished to see what the result would be with ordinary sewer gas from a very foul soil pipe, both with gases and with germs, and he therefore experimented on the trap of a pan closet, the kind generally used in Glasgow at the time. The conditions were as shown in Fig. 40. Two other pan closets entered the soil pipe, which was ventilated above the upper closet by a 2-in. pipe up to and through the roof. The lower end of the soil pipe passed untrapped and unventilated into the drain. The mouth of the sewer was submerged at high tide so that sewer gases tended to be pressed back toward the houses, so that as trying conditions as possible were presented.

Fig. 39. Apparatus for determining: whether bacteria can pass through water in traps.

Fig. 39. Apparatus for determining: whether bacteria can pass through water in traps.

Dr. Carmichael took off the basin and pan from the water-closet and screwed a zinc plate air-tight over the receiver, as shown. Through this plate passed two lead pipes with stopcocks extending about eight inches above the plate.

Fig. 40. Apparatus for determining if gases and germs from a foul soil pipe can pass through trap seals.

Fig. 40. Apparatus for determining if gases and germs from a foul soil pipe can pass through trap seals.

The sewer air from the receiver was drawn by an aspirator through six glass vessels of water arranged in a row, as shown. The air was introduced into the receiver from the room after having been washed by passing through the liquid in the single glass vessel at the left. The water in all the vessels was charged with a strong solution of caustic potash which is capable of absorbing carbonic acid, the first substance to be sought for in the sewer air. The aspiration was continued for twenty-four hours in each of a large number of tests made for the gas. The average amount of carbonic acid which passed through the trap seal in twenty-four hours was a little over 7 grains.

For the ammonia testings sulphuric acid was used in the water of the seven vessels instead of potash. The amount of this gas which passed through the trap in 24 hours was 1-400th to 1-20oth of a grain. The amount of sulphuretted hydrogen in the same time was about 1-100th of a grain.

'These are the quantities," says Dr. Carmichael, "of the only sewage gases existing in the soil pipe, in estimable quantities, which pass through an ordinary water closet trap in 24 hours."

The doctor made another series of tests with a new lead W. C. trap, as shown in Fig. 41, so as to make certain that the gases found were in no way peculiar to the filth in the old trap. Almost identically the same results were obtained, but the amounts of ammonia and sulphuretted hydrogen were very slightly less, which might show that the filth in the receiver and old trap may have been responsible for a small percentage of the gas produced.

He also made the experiments again with the soil pipe 2-inch vent closed at the top. This increased the amount of gases passed through the water seal to 32 and 17 grains in the case of the carbonic acid. The sulphuretted hydrogen was increased from 1-100th to 1-90th of a grain in the 24 hours. Diffused through the atmosphere of a house these quantities are from a health point of view absolutely harmless. 'Thirty-two grains (the largest quantity) of carbonic acid is less than the quantity of the same gas given off when a bottle of lemonade is drawn." A man exhales in the same time about 400 times the amount which passed through the trap from an unventilated and very foul soil pipe. The 1-100th of a grain of ammonia and the 1-60th of a grain of sulphuretted hydrogen would, of course, be utterly unnoticeable. As for the foetid organic vapors, mere traces of these exist in the sewer-air, and if they do pass through the trap they would be in quantities too minute for detection. "We are able, however, to state in more exact terms something as to the quantity in which they may come through. These vapors are organic; as already stated, they are carbo-ammoniacal, they are therefore decomposed by Wanklyn's process for the estimation of nitrogenous matter, as ammonia, and are, therefore (if they do pass through the trap), included in the ammonia; and consequently less (probably very much less) than the 1-100th of a grain in 24 hours. This, I need scarcely say, must be harmless."

Fig. 41. Apparatus for determining if gases and germs from a clean soil pipe can pass through water in trap.

Fig. 41. Apparatus for determining if gases and germs from a clean soil pipe can pass through water in trap.