=6. Electricity.=--Electricity, though not in itself injurious to bacteria, is used as an indirect means for destroying bacteria in a practical way. This is done by electrical production of some substance which is destructive to bacteria as in ozone water purification (Petrograd, Florence, and elsewhere), or the use of ultra-violet rays for the same purpose (Marseilles, Paris) and for treatment of certain disease conditions. Electricity might be used as a source of heat for disinfecting purposes should its cheapness justify it. It has also been used in the preservation of meats to hasten the _penetration of the salt_ and thus reduce the time of pickling. Electrolyzed sea water has been tried as a means of flushing and disinfecting streets, but it is very doubtful if the added expense is justified by any increased benefit. A number of electric devices have been put forth for various sterilizing and disinfecting purposes and doubtless will continue to be, but everyone should be carefully tested before money is invested in it.[19] [Illustration: FIG. 102.--An electric milk purifier (pasteurizer). The milk flowing from cup to cup completes the circuit when the current is on. The effect is certainly a heat effect. Sparking occurs at the lips of the cups.] [Illustration: FIG. 103.--One of the ten filter beds of the Columbus water filtration plant with the filtering material removed. Sand is the filtering material. All of the beds together have a capacity of 30,000,000 gallons daily.] [Illustration: FIG. 104.--Suction filtration. _A_, Berkefeld filter in glass cylinder containing the liquid to be filtered; _B_, sterile flask to receive the filtrate as it is drawn through; _C_, water pump; _D_, manometer, convenient for detecting leaks as well as showing pressure; _E_, bottle for reflux water.] [Illustration: FIG. 105.--Pressure filtration. _A_, cylinder which contains the filter candle; _B_, cylinder for the liquid to be filtered; _C_, sterile flask to receive the filtrate; _D_, air pump to furnish pressure.]