=7. Filtration.=--Filtration is a process for rendering fluids sterile by passing them through some material which will hold back the bacteria. It is used on a large scale in the purification of water for sanitary or manufacturing reasons (Fig. 103). Air is also rendered “germ free” in some surgical operating rooms, “serum laboratories” and breweries by filtration. In the laboratory it is a very common method of sterilizing liquids which would be injured by any other process. The apparatus consists of a porous cylinder with proper devices for causing the liquid to pass through either by suction (Fig. 104) where the pressure will be only one atmosphere (approximately 15 pounds per square inch), or by the use of compressed air at any desired pressure (Fig. 105). The two main types of porous cylinders (“filter candles,” “bougies”) are the Pasteur-Chamberland (Fig. 106) and the Berkefeld. The former are made of unglazed porcelain of different degrees of fineness, the latter of diatomaceous earth (Fig. 107) The Mandler filter of this same material is now manufactured in the United States and is equal if not superior to the Berkefeld. The designs of complete apparatus are numerous. [Illustration: FIG. 106.--Pasteur-Chamberland filter candles about one-half natural size.] =8. Burying.=--This is a time-honored method of disposing of infected material of all kinds and at first thought might not be considered a means of _disinfection_. As a matter of fact, under favorable conditions it is an excellent method. The infected material is removed. Pathogenic organisms tend to die out in the soil owing to an unfavorable environment as to temperature and food supply, competition with natural soil organisms for what food there is, and the injurious effects of the products of these organisms. Care must be taken that the burial is done in such a way that the _surface_ soil is not contaminated either directly or by material brought up from below by digging or burrowing animals, insects, worms, or movement of ground water to the surface. Also that the underground water supply which is drawn upon for use by men or animals is not contaminated. Frequently infected material, carcasses of animals, etc., are treated in some way so as to aid the natural process of destruction of the organisms present, especially by the use of certain chemical agents, as quicklime (see p. 158). [Illustration: FIG. 107.--Berkefeld filter candles about one-half natural size.] CHAPTER XIV. DISINFECTION AND STERILIZATION (CONTINUED). CHEMICAL AGENTS. A very large number of chemical substances might be used for destroying bacteria or preventing their growth either through direct injurious action or by the effect of concentration. Those which are practically useful are relatively few, though this is one of the commonest methods of disinfecting and the word “disinfectant” is frequently wrongly restricted to chemical agents.