10 STERILIZATION BY MOIST HEAT 



itself is supported within a cylindrical sheet-iron or copper furnace provided 

 with one or two rings of Bunsen burners. 



Technique. (a) Pour sufficient water into the boiler to reach to just 

 below the bottom of the wire basket ; distilled water is preferable, as by 

 its use furring is avoided. 



Place the apparatus to be sterilized in the basket, and lay two or three 

 thicknesses of cloth or paper over the wool plugs to prevent condensation 

 water dropping on to them from the cover. 



(b) Adjust the india-rubber washer, put on the cover, and screw up the 

 bolts with the fingers. It is better to use the fingers than the key provided 

 with the autoclave, because with the latter an unnecessary amount of force 

 is very likely to be applied, with the result that the washer is quickly ruined ; 

 moreover, careless manipulation with the key will soon strip the screws. 



(When the autoclave is not in use, the bolts should remain loosened, and the 

 washer removed and hung up because if left under the cover it gets crushed.) 



(c) Open the steam tap. 



(d) It will be sufficient to light one ring of burners. Hold the taper to the 

 burner before turning on the gas, and take special notice that the burners 

 do not light below : should this happen, turn out the gas and re-light it. 



(e) As soon as the water begins to boil, steam will escape from the tap 

 in the cover, and must be allowed to continue to do so until the pressure of 

 the steam within causes it to issue with a whistling sound in a powerful and 

 continuous jet. 



The object of this manoeuvre is to expel the whole of the air from the interior 

 of the autoclave, since if any air remain in the boiler the manometer readings will 

 not be reliable. Still however much care be taken it is impossible to drive out 

 all the air, and the larger the autoclave the larger will be the volume of air remaining. 

 A more effectual means of expelling the air is to compress and decompress repeatedly 

 by opening and shutting the steam tap, but this method should never be adopted 

 when sterilizing fluids because under the influence of a sudden lowering of the 

 pressure the plugs and contents of the flasks and tubes are driven out by the violent 

 boiling of the liquids. 



Now close the steam tap. The pressure and temperature will rise rapidly, 

 and when the manometer records the temperature required (115-120 C.), 

 lower the gas and regulate it by trial until the manometer reading is steady. 

 Continue the heating at this temperature for 20 minutes. 



(/) When sterilization is completed, turn out the gas ; the manometer 

 needle soon falls to zero, and then, but not until then, open the steam tap. 

 When all the steam has escaped unscrew the bolts, raise the cover, and remove 

 the contents. If the plugs be damp it is well to put the flasks, etc., in the 

 incubator until the wool dries. 



The following minor practical details in the working of an autoclave may be 

 mentioned. It is important never to open the steam tap until the pressure within 

 the apparatus has fallen to the zero mark on the manometer, for the reason already 

 given, namely that under the influence of sudden decompression the fluid contents 

 of the flasks, etc., are liable to be discharged into the autoclave. Again, to avoid 

 accidents by scalding from an escape of steam beneath the cover, the steam 

 tap must always be opened before the bolts are loosened. Lastly, to obviate any 

 difficulty in lifting the cover owing to the rubber washer sticking to the metal, 

 always open the autoclave before the latter gets quite cold. 



Note. The autoclave is also available for sterilization at 100 C. The 

 procedure will be the same as regards the first four steps a, b, c, d, but the 

 steam tap must be left open the whole time (30 minutes), and the gas 

 burners regulated so that the pressure as recorded by the manometer needle 

 does not rise above the zero point. 



