He further reports that in his experiments the effluents contained on an average 100 milligram per litre more free carbonic acid than the raw sewage, and that the quantity contained in the effluents represents only a small portion of the total amount of carbonic acid formed during the whole process. The by far greatest portion of carbonic acid escapes into the air. Concerning the air in the pores of the filling material during periods of aeration, Dunbar states that it contains sometimes not less than from 6 to 10 per cent. carbonic acid. [Sidenote: Nitrogen escapes in gas form into the air.] (_g_) _Nitrogen._—It is quite clear from all experiments that a considerable amount of the total nitrogen contained in raw sewage is abstracted by the filling material of intermittent contact beds, and it is interesting to ascertain what becomes of it! Does it accumulate in the bed? In that case, one has a right to assume that the satisfactory work of the bed would gradually cease! As this is, however, not the case, and as, further, the sludge formed in the bed, whether it be fairly fresh or very stale, only contains a very small amount of total nitrogen, we must surmise that the nitrogen after its retention by the bed escapes in gas form—like the carbonic acid—into the atmosphere. [Sidenote: The presence of nitric acid is not an unfailing guide for determining the satisfactory character of the effluent.] (_h_) _The Formation of Nitric Acid._—Concerning the presence of nitric acid in the effluents from intermittent contact beds, Dunbar is of opinion that it offers certain means for forming an opinion of the processes taking place in the same, but that it is only in a subordinate sense an indication of the degree of purification attained and must not be taken as an unfailing guide for determining the satisfactory character of the effluent. [Sidenote: Nitrifying bacteria always present in town’s sewage.] [Sidenote: Nitric acid is formed very rapidly, but only during periods of rest.] Nitrifying bacteria are always present in ordinary town’s sewage, but it would appear that other micro-organisms besides Winogradsky’s bacteria assist in the process of nitrification. Nitric acid is formed very rapidly, but only during periods of rest, and besides aeration other less powerful influences are at work. [Sidenote: Reduction of nitric acid when bed is filled from bottom with an upward flow.] It is further interesting to note that, according to Dunbar, the greater portion of nitric acid which has been formed during periods of aeration becomes completely reduced in a very short time, when the bed is filled with an upward flow from the bottom, and that only a small portion remains in the form of nitrous acid. 3. ARTIFICIAL SELF-PURIFICATION OF SEWAGE IN SEPTIC TANKS. [Sidenote: Septic tanks only used in combination with contact beds.]