Now, however, it is recognised that the second portion of the chemical action is vastly the more important, indeed, the only vital, part of the process. This is the chemical effect of the layer of scum and mud on the surface of the sand at the top of the filter-bed. The mechanical part of this layer is, of course, the holding back of the particulate matter which has not subsided in the reservoir; the vital action consists in what is termed _nitrification_ of unoxidised substance, which is accomplished in this layer of organic matter. We shall deal at some length with the principles of nitrification when we come to speak of soil. But we may say here that by nitrification is understood a process of oxidation of elementary compounds of nitrogen, by which these latter are built up into stable bodies which can do little harm in drinking water. From what has been said it will be seen that the action of a filter-bed is of a complicated nature. There is (1) subsidence of the grosser particles of impurity in the water; (2) mechanical obstruction to impurities in the interstices of the scum, sand, and gravel in the filter; (3) oxidation of organic matter by the oxygen held in the pores of the sand and gravel; (4) nitrification in the vital scum layer, which is accomplished by micro-organisms themselves. This latter is now considered to be incomparably the most important part of the filter. That being so, its removal, except when absolutely necessary, is to be avoided as detrimental to the efficiency of the filter. New filters have obviously but little of this action. Hence it is wise to allow a new filter-bed to act for a short period (say twenty-four to forty-eight hours) before the filtered water is used for domestic purposes, in order to allow the organic layer to be formed. This must also be borne in mind after a filter-bed has been cleaned.
To maintain this nitrifying action of a filter in efficiency, Koch suggested, in the second place, _that the rate of filtration must not exceed four inches per hour_. At the Altona water-works this rate of filtration was maintained, and the number of organisms always remained below 100 per cc., which, as we have seen, is the standard. Thirdly, it is important that _periodic bacteriological examinations should be made_. Koch's emphasis upon this point is well known, and the cumulative experience of bacteriologists only further supports such a course being taken. If it be true that efficient sand filtration is a safeguard against pathogenic germs like typhoid and cholera, then there can be but one criterion of efficiency, viz., their absence in the filtered water, which can only be ascertained by regular examination. But it is not alone for pathogenic germs that filtration is proposed. Filtered water containing more than 100 micro-organisms of any kind per cc. is below the standard in purity, and should on no account be distributed for drinking purposes. In this country chemical analysis, with a more or less cursory microscopic examination, has been almost invariably accepted as reliable indication of the condition of the water. But such an examination is not really any more a fair test of the working of the filter than it is of the actual condition of the water. It is true, the quantity of organic matter can be estimated and the condition in which it exists in combination obtained; but it cannot tell us what a bacteriological examination can tell us, viz., the quantity and quality of living micro-organisms present in the water. Upon this fact, after all, an accurate conclusion depends. There is abundant evidence to show that no valuable opinion can be passed upon a water except by both a chemical and a bacteriological examination, and further by a personal investigation, outside the laboratory, of the origin of the water and its liabilities to pollution.