medium is an absolutely essential condition in all filters.” [Illustration: 9. 10. Atkins’s filters] Fig. 9 shows Atkins’s filter, in which _a_ is the unfiltered and _b_ the filtered water, _c_ being a block of charcoal formed by mixing powdered charcoal with pitch or resin, moulding and calcining. The filter is capable of being taken to pieces and can thus be easily and frequently cleaned. The block should on such occasions be scraped, washed, boiled, and baked. Fig. 10 illustrates another form of Atkins’s, in which powdered charcoal is used, retained between movable perforated earthenware plates. [Illustration: 11. 12. Sawyer’s Filters.] Figs. 11, 12 represent Sawyers filters, in which _a_ is unfiltered water; _b_, filtered water; _c_, charcoal hollow cone; _d_, filtered water tap; _e_, sediment tap; _f_, mass of granular charcoal. The most important feature here is the _upward_ filtration. Charcoal modified.--Several substances have been proposed for combination with carbon to improve its filtering capacity or increase its germ-destroying powers. [Illustration: 13. Silicated Carbon. 14. Silicated Carbon.] Silicated Carbon.--This was one of the earliest modifications of the simple carbon block. Figs. 13, 14 show respectively the forms adopted for downward and upward filtration. In the former, the stoneware receptacle is divided into two parts by a diaphragm upon which there is fixed, by a porcelain stay, a silicated carbon block, which entirely closes the apertures in the diaphragm. The upper surface and corners of the filtering block are non-porous, consequently the water has to enter at the edges and follow the course indicated by the arrows, before it can reach the clear water compartment below. In cleaning the filter, it is only necessary to unscrew the nut, when the block can be lifted out and soaked in boiling water, after which the surface can be scrubbed. The ‘Army Medical Report’ says of filters employing carbon in porous blocks that “These are powerful filters at first, but they are apt to clog, and require frequent scraping, especially with impure waters. Water filtered through them and stored, shows signs of the formation of low forms of life, but in a less degree than with the loose charcoal. After a time, the purifying power becomes diminished in a marked degree, and water left in contact with the filtering medium is apt to take up impurity again, though perhaps in a less degree than is the case with the loose charcoal.” The advantages of combining silica with the carbon are not at first sight apparent. [Illustration: 15. Maignen’s Filter.]