It ought further to be pointed out that the absorbing powers of the filling material gradually increase until the bed has become ripe. This fact was formerly stated to be due to the development of the proper micro-organisms within the bed, but it would seem to be chiefly due to the slimy surface coating of the particles of the filling material, or spongy bacterial growth, as it has frequently been called, which does not only assist mechanical filtration but also possesses high powers of absorbing oxygen. [Sidenote: Absorbing powers soon cease in the absence of micro-organisms and air.] But it cannot be open to doubt that the absorbing powers of the filling material are dependent in some way or other on the presence of micro-organisms, for Dunbar has shown that in the absence of micro-organisms and without periods of aeration these powers soon cease. [Sidenote: Oxygen is absorbed from air in the pores with great energy.] (_e_) _Consumption of Oxygen by the Filling Material._—The oxygen necessary for the proper work of an intermittent contact bed is abstracted with great energy from the atmospheric air, with which the pores become filled during periods of rest. Through diffusion, and through the vacuum created by the processes of absorption, further quantities of oxygen are taken from the atmospheric air, even under difficult conditions, and, as pointed out in the Manchester report for the year ending 27th March, 1901, “there is, therefore, little need to force air into a bed.” [Sidenote: The oxygen taken up during aeration is not imparted to the sewage at the next filling and does not escape in the effluent.] The oxygen thus taken up is not imparted in gas form to the sewage during the next filling, and the effluents from intermittent contact beds are not saturated with oxygen. Dunbar states that the effluents of a satisfactorily worked bed frequently only contain one cubic centimetre of free oxygen per litre. Clowes reports a similar result in his third report on the London experiments. [Sidenote: The greatest quantity of oxygen is consumed during the oxidation of the products formed by micro-organisms.] There can be no doubt that by far the greatest quantity of oxygen is consumed during the process of oxidation of the products formed by micro-organisms from putrescible organic substances. [Sidenote: Consumption of oxygen and formation of carbonic acid not solely due to biological agencies.] (_f_) _Formation of Carbonic Acid._—Dunbar has shown by his experiments that the consumption of oxygen and the formation of carbonic acid is not solely due to biological agencies, but is to some extent the result of physico-chemical processes. [Sidenote: More free carbonic acid contained in the effluent than in the raw sewage.] [Sidenote: By far the greatest portion of carbonic acid escapes into the air.]