The waste of fixed baths is almost clean, containing nothing but a little soap at most; bedroom waste contains soap and urine, but no solid particles of any size to give trouble, except a stray bit of paper, or an old match, or a few hairs, and some fluff from towels, which will all be caught upon the perforated tiles, and can be swept up occasionally. [Illustration: Fig. 28.—Kitchen Sink with Duplicated Outside Filter.] The waste from kitchen and pantry sinks needs careful straining and filtering before it is allowed to flow into an open gutter. I have mentioned the necessity of providing strainers and a sink-basket, and I now proceed to describe the slop-filter which is advisable for the kitchen sink (fig. 28). The waste-pipe of the sink must terminate 2 feet or 2 feet 6 inches above the ground level, and be provided with a reversible nozzle delivering over a filtering vessel made of concrete or iron. This filter is in duplicate, and is provided with a diaphragm reaching to within an inch of the bottom. Each half of the vessel measures 1 foot by 1 foot 6 inches, and is 2 feet 6 inches deep, and has a capacity of 3·75 cubic feet. The outflow is immediately beneath the diaphragm, and empties into the open gutter. Each half of the filter is filled with stones varying in size from a hazel-nut to a walnut, and the waste is allowed to flow through one half of the filter, and then, when that half gets foul, the nozzle is reversed, and the second half is brought into use, and the half first used can be cleaned out. The filter must be provided with a fine copper strainer, and if the slops be carefully strained the filter will not get foul for months; but if lumps of fat and slabs of cabbage-leaves be allowed to get into the filter, it soon gets foul, as does the abomination known as a fat-trap. The method of purifying sewage by 'intermittent downward filtration' is well understood, and the methods advocated here are merely modifications of what has been done in this country, and also by the Massachusetts Board of Health. What is meant by 'intermittent downward filtration'? How frequent are the intermissions?