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test-tubes stopped with a cotton-wool plug remain sterile indefinitely.

I. Steam Sterilization.

For objects liable to be injured by dry heat, such

FIG. 22.

с

Kitasato's fil

ter: a, porcelain bougie; b, attachment for suctionpump; c, reservoir; d,

sterile receiver.

as culture-media, fluids, instruments, etc.:

(a) Fill the water-tank of the sterilizer1 and start the flame.

(b) When the chamber is filled with steam place the objects to be sterilized within and close the door.

(c) If the objects will not be injured by prolonged heating, allow them to remain for one hour.

(d) If prolonged heating is injurious, allow them to remain twenty minutes, and process repeat the three times at intervals of twenty-four hours.

II. Steam Sterilization under Pressure.

For rapid and effective sterilization. method an "autoclave" is used.

In this

(a) Adjust the safety-valve at the desired pressure, say thirty pounds.

See that an abundance of water is present, so that the steam will not be superheated.

1 An "Arnold" sterilizer is the form usually employed.

(b) Place the objects to be sterilized in the chamber.

(c) Close the door and turn on the steam, first allowing the air present in the chamber to escape. (d) Bring the temperature to, say 120° C.; allow it to remain there fifteen minutes.

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FIG. 23.-Arnold steam sterilizer, Boston Board of Health

pattern.

(e) Shut off the steam and allow the apparatus to cool well below 100° C. before opening the door or allowing the steam to blow off.

III. Hot-air Sterilization.

For glassware and other objects not liable to be

injured by dry heat. The oven of an ordinary gasstove into which a thermometer can be inserted makes an excellent hot-air sterilizer.

(a) Wash thoroughly, using, if necessary, the cleaning mixture recommended on page 14, all

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FIGS. 24, 25.-Autoclaves for rapid sterilization by steam under pressure.

flasks, test-tubes, Petri dishes, etc., to be used in the preparation of culture-media.

(b) Plug the flasks and test-tubes with cotton."

"Sliver" obtained from cotton-mills is excellent for plugging test-tubes, etc.

In the case of the test-tubes, the plugs should be tight enough so that the tubes can be lifted by them.

(c) Place in the hot-air sterilizer, close the door, and bring the temperature to 150° C.; keep it there

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for three-quarters of an hour, or until the cotton plugs begin to turn brown.

(d) Allow the oven to cool before opening the door. Place the glassware in a clean place free from dust until used.

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