A tube _a_ (Fig. 16) of unglazed porcelain is enclosed in another _b_ of metal, and the water to be filtered is admitted to the space between the two by turning a stop-cock. Thence it slowly filters through to the inside of the porcelain tube, and flows out at the bottom. Under a pressure of 2 atmospheres, or 30 lb. to the sq. in., a tube 8 in. in length, with a diameter of 1 in., will yield about 5 gal. of water daily. For a larger supply, it is only necessary to increase the size or the number of the tubes. In cleansing the filter, the porcelain tube is removed, and the microbes and other matter that have accumulated on the outer face of it are brushed off. The tube may also be plunged in boiling water in order to destroy any germs that may be supposed to have penetrated beneath its surface; or it may be heated in a gas jet or in a furnace. In fact, it can be more readily and more thoroughly cleaned than most of the domestic filters in ordinary use. It is interesting to remark that some of the earliest filtering vessels of which we have any knowledge are simply made of porous earthenware. After all our modern researches after antiseptic filtering media, we are reverting to the ways of our remotest forefathers. Filtering Cisterns.--The following is a description of a filter which purifies foul water from organic impurities held in solution as well as from suspended solids. Take any suitable vessel with a perforated false bottom, and cover it with a layer of animal charcoal, on the top of that spread a layer of iron filings, borings, or turnings, the finer the better, mixed with charcoal dust; on the top of the filings place a layer of fine clean siliceous sand, and you will have a perfect filter. Allow the foul water to filter slowly through the above filter, and you will produce a remarkably pure drinking-water. Before placing the iron filings in the filter, they must be well washed in a hot solution of soda or potash, to remove oil and other impurities, then rinse them with clean water; the filings should be mixed with an equal measure of fine charcoal. If the water is very foul, it must be allowed to filter very slowly. The deeper the bed of iron filings is the quicker they will act.