It can be seen by this brief explanation that it is not possible to assign any particular rate of application to any particular kind of treatment, since in all the methods of purification which have been worked out considerable variation in the details of that particular method have been practised. It will be possible, therefore, in succeeding chapters to indicate by the size of filters recommended only limiting or average values for rates of purification, since those rates are always dependent upon other factors than the particular method being discussed. It must also be remembered that soils may exist which have no porosity whatever, and through which it is impossible for sewage to make its way. Such soils are not available for sewage purification, and, no matter how small the rate or how careful the method of application, such areas will fail to produce any practical purification. Soils like clay, peat, and fine water-deposited silt are of this sort. Clay soils may sometimes become pulverized by cultivation so that they will ultimately be able to take care of a moderate amount of sewage. In such a case it is possible to dispose of sewage successfully in the top six inches of soil which, by continual cultivation, has been made out of the stiff clay. In such cases, the difficulty is not that of oxidizing the sewage, but that of taking care of the effluent, which must be held between the cultivated soil and the raw clay underneath. The second requirement mentioned is secured by discharging the sewage onto the soil area at intervals, the number of doses per day depending upon the size of particles in the bed. There has been a general principle established that the size of these doses ought to be smaller as the size of the particles increases, so that, whereas in the case of sand beds the total daily dose is usually divided into from one to three parts and each part delivered onto the bed with an appropriate interval, in the case of coarser materials used for sprinkling filters, the time interval between doses is much reduced and in some installations recently constructed in England that interval has been measured in seconds. The variations in the rate of flow of sewage onto any filter, however, are so great that any such requirement as designing discharging apparatus to work at intervals of a few seconds is useless, and if as small an interval as one minute is provided for the coarsest material for the maximum rate of flow at any time of the day, the installation will probably be successful for the lesser rates occurring at other times of the day. As an indication of the way in which this modification is made, it is customary, when the size of soil particles is that of peas, to make the interval between successive discharges about one hour, so that the dose applied at any one time would be equal to 1/24 of the daily volume. With gravel filling, the particles being the size of English walnuts, the interval between doses is shortened to five