3. Sterile filter flask of suitable size, for the reception of the filtered fluid, its mouth closed by a cotton-wool plug. 4. Water injector Sprengel (see Fig. 38, c) pump, or Geryk's pump (an air pump on the hydraulic principle, sealed by means of low vapor-tension oil, Fig. 35). If this latter is employed, a Wulff's bottle, fitted as a wash-bottle and containing sulphuric acid, must be interposed between the filter flask and the pump, in order to prevent moist air reaching the oil in the pump. 5. Air filter (_vide_ page 40) sterilised. 6. Pressure tubing. 7. Screw clamps (Fig. 36). METHOD.-- 1. Couple the exhaust pipe of the suction pump with the lateral tube of the filter flask (first removing the cotton-wool plug from this latter), by means of pressure tubing, interposing, if necessary, the wash-bottle of sulphuric acid. [Illustration: FIG. 34.--Porcelain filter candle.] [Illustration: FIG. 35.--Geryk air pump.] 2. Remove the cotton-wool plug from the neck of the filter flask and adjust the porcelain candle in its place. [Illustration: FIG. 36.--Screw clamps.] 3. Attach the nozzle of the separatory funnel to the filter candle by means of the perforated rubber stopper (Fig. 37). [Illustration: FIG. 37.--Apparatus arranged for filtering--aspiration.] 4. Open the tap of the funnel, and exhaust the air from the filter flask and wash-bottle; maintain the vacuum until the filtration is complete. 5. When the filtration is completed close the tap of the funnel; adjust a screw clamp to the pressure tubing attached to the lateral branch of the filter flask; screw it up tightly, and disconnect the acid wash-bottle. 6. Attach the air filter to the open end of the pressure tubing; open the screw clamp gradually, and allow filtered air to enter the flask, to abolish the negative pressure. 7. Detach the rubber tubing from the lateral branch of the flask, flame the end of the branch in the Bunsen, and plug its orifice with sterile cotton-wool. 8. Remove the filter candle from the mouth of the flask, flame the mouth, and plug the neck with sterile cotton-wool. 9. Disinfect the filter candle and separatory funnel by boiling. If it is found necessary to employ pressure in addition to or in place of suction, insert a perforated rubber stopper into the mouth of the separatory funnel and secure in position with copper wire; next fit a piece of glass tubing through the stopper, and connect the external orifice with an air-pressure pump of some kind (an ordinary foot pump such as is employed for inflating bicycle tyres is one of the most generally useful, for this purpose) or with a cylinder of compressed air or other gas.