(b) _Porcelain Filter._--The sterilisation of liquids by filtration is effected by pa.s.sing them through a cylindrical vessel, closed at one end like a test-tube, and made either of porous "biscuit" porcelain, hard-burnt and unglazed (Chamberland system), or of Kieselguhr, a fine diatomaceous earth (Berkefeld system), and termed a "bougie" or "candle"

(Fig. 34).

NOTE.--In selecting candles for use in the laboratory avoid those with metal fittings, since during sterilisation cracks develop at the junction of the metal and the siliceous material owing to the unequal expansion.

In this method the bacteria are retained in the pores of the filter while the liquid pa.s.ses through in a germ-free condition.

It is obvious that to be effective the pores of the filter must be extremely minute, and therefore the rate of filtration will usually be slow. Chamberland filter candles possess finer channels than Berkefeld candles and consequently filter much more slowly. To overcome this disadvantage, either aspiration or pressure, or a combination of these two forces, may be employed to hasten the process.

Doultons white porcelain filters it may be noted are as efficient as the Chamberland candles and filter rather more rapidly.

_Apparatus Required._--

1. Separatory funnel containing the unfiltered fluid.

2. Sterile filter candle (Fig. 34), the open end fitted with a rubber stopper (Fig. 34, a) perforated to receive the delivery tube of the separatory funnel, and its neck pa.s.sed through a large rubber washer (Fig. 34, b) which fits the mouth of the filter flask.

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.

[Ill.u.s.tration: FIG. 34.--Porcelain filter candle.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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 gla.s.s 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.

In order to filter a large bulk of fluid very rapidly it is necessary to use a higher pressure than gla.s.s would stand, and in these cases the metal receptacle designed by Pakes (Fig. 38, a), to hold the filter candle itself as well as the fluid to be filtered, should be employed.

(A vacuum must also be maintained in the filter flask, by means of an exhaust pump, during the entire process.)

This piece of apparatus consists of a bra.s.s cylinder, capacity 2500 c.c., with two shoulders; and an opening in the neck at each end, provided with screw threads.

A nut carrying a pressure gauge fits into the top screw; and into the bottom is fitted a bra.s.s cylinder carrying the filter candle and prolonged downwards into a delivery tube. Leakage is prevented by means of rubber washers.

Into the top shoulder a tube is inserted, bent at right angles and provided with a tap. All the bra.s.s-work is tinned inside (Fig. 38, a).

In use the reservoir is generally mounted on a tripod stand.

~To Sterilise.~--

1. Insert the filter candle into its cylinder and screw this loosely on.

[Ill.u.s.tration: FIG. 38.--Pakes' filtering reservoir--pressure and aspiration.]

2. Wrap a layer of cotton-wool around the delivery tube and fasten in position.

3. Remove the nut carrying the pressure gauge and plug the neck with cotton-wool.

4. Heat the whole apparatus in the autoclave at 120 C. for twenty minutes.

METHOD.--

1. Remove the apparatus from the autoclave, and allow it to cool.

2. Screw home the box carrying the bougie.

3. Set the apparatus up in position, with its delivery tube (from which the cotton-wool wrapping has been removed) pa.s.sing through a perforated rubber stopper in the neck of a filter flask.

[Ill.u.s.tration: FIG. 39.--Closed candle arranged for filtering.]

4. Fill the fluid to be filtered into the cylinder and screw on the nut carrying the pressure gauge. (This nut should be immersed in boiling water for a few minutes previous to s.c.r.e.w.i.n.g on, in order to sterilise it.)

5. Connect the horizontal arm of the entry tube with a cylinder of compressed oxygen (or carbon dioxide, Fig. 38, b), by means of pressure tubing.

6. Connect the lateral arm of the filter flask with the exhaust pump (Fig. 38, c) and start the latter working.

7. Open the tap of the gas cylinder; then open the tap on the entry tube of the filter cylinder and raise the pressure in its interior until the desired point is recorded on the manometer. Maintain this pressure, usually one or one and a half atmospheres, until filtration is completed, by regulating the tap on the entry tube.

Some forms of filter candle are made with the open end contracted into a delivery nozzle, which is glazed. In this case the apparatus is fitted up in a slightly different manner; the fluid to be filtered is contained in an open cylinder into which the candle is plunged, while its delivery nozzle is connected with the filter flask by means of a piece of flexible pressure tubing (previously sterilised by boiling), as in figure 39.