A further silting-up agency is the slimy surface coating of the particles of the filling material. This accumulation is well known to all who have had to do with intermittent contact beds, and has been described as spongy bacterial growth. The Manchester report for the year ending March 27, 1901, contains on page 62 the following passage: “This (spongy bacterial growth) is at once the cause of increased efficiency in the bed and loss of capacity. On examining the material of a contact bed in active condition, every piece is seen to be coated over with a slimy growth. If this is removed it soon dries to a stiff jelly, which can be cut with a knife. Under the microscope masses of bacteria and zoogloea will be found to be present.” [Sidenote: Accumulations of decomposed substances in the pores.] In addition to this slimy surface-coating of the particles, there are also found in the pores, especially in the upper layers of the filling material—and in fine beds more so than in a coarse bed—accumulations which are “akin to humus or garden soil.” They contain to a limited extent only putrescible substances, and appear to be the remains of organic matter decomposed by the activity of micro-organisms. [Sidenote: Periods of rest will not permanently restore portion of the original water capacity of the bed.] It was formerly maintained with considerable persistency that periods of rest would permanently restore to a systematically worked bed a portion of its lost water capacity, but such a contention has been proved to be wrong. It is quite true that immediately after periods of rest an increase of the water capacity is very noticeable, which is probably due to drying up processes within the bed during the rest, but such an increase is not permanent and is lost again more or less quickly; it is therefore only temporary and not permanent. Where, however, a bed has not been systematically worked, i.e. where it has been worked at a greater rate than is suitable, and where in consequence of this a large quantity of undecomposed substances is stored in it, a period of rest may permanently restore a portion of the lost capacity; but this is due to the mineralisation of these undecomposed organic substances during the rest. It follows from these remarks, as has been stated above, that when the organic substances are regularly decomposed during systematic work a period of rest cannot materially affect the water capacity, and that where a considerable permanent restoration of the water capacity takes place the bed has not been properly worked. [Sidenote: Decrease of capacity is accompanied to some extent by increase of efficiency and _vice versa_.] It would, however, be incorrect to assume that the silting up of the bed affects its efficiency besides reducing the capacity. On the contrary! To some extent decrease of capacity is accompanied by increase of efficiency and _vice versa_! [Sidenote: Higher capacity of beds in summer than in winter.]