8 



STERILIZATION BY MOIST HEAT 



This process embodies the principle of sterilization by discontinuous heating. 

 The explanation put forward by Tyndall was that the hay infusion contains both 

 bacilli and spores (B. subtilis). By heating to 100 C. the bacilli, but not the 

 spores, are killed. The latter germinate as the fluid cools, and are killed during the 

 second heating. A few spores however escape destruction on the second heating ; 

 these will have germinated by the time the third heating is due. After the third 

 heating then sterilization is completed. The explanation now given however is 

 that the resistance of micro-organisms is gradually lowered under the influence of 

 repeated heating. 



Steamers. 



1. Koch's steamer. Koch's steamer (fig. 4) consists of a cylindrical 

 copper boiler, provided with a water gauge below and closed above by a lid 

 through a hole in which a thermometer can be passed. It is fitted with 



perforated and moveable metal 

 trays on which to rest the appar- 

 atus. 



A metal cylinder open at both 

 ends is often supplied with the 

 sterilizer, so that the latter can 

 be lengthened when necessary by 

 fitting the metal cylinder on top. 

 Technique. When sterilizing 

 culture media by steam at 100 C., 

 it is advisable to use vessels already 

 sterilized in the hot air sterilizer. 

 (a) Pour sufficient water into 

 the steamer to reach the level 

 marked on the water gauge. Stand 

 the vessels on the trays, and if 

 extra space be needed adjust the 

 lengthening cylinder. Put on the 

 lid, and insert the thermometer. 



(6) Light the gas under the 

 boiler, note when steam begins to 

 escape from under the lid the 

 thermometer will then register 

 98-100 C. and maintain the 

 apparatus at this temperature for 

 30 minutes. 



(c) Heat again in a similar manner on the two following days. 

 When the flasks, tubes, etc., are taken out of the steamer, the wool plugs 

 are generally wet with water of condensation ; and since wool is only efficient 

 as a filter for micro-organisms so long as it is absolutely free from moisture, 

 the vessels may be put in the incubator for an hour or two to dry the 

 plugs. 



In many of the newer patterns of steamers the steam circulates between 

 double walls before escaping, thus maintaining an absolutely constant tem- 

 perature in the steamer. Some forms are further provided with a constant- 

 level adjustment. [One of the most useful of these newer patterns is that 

 made by Hearson.] 



[2. Hearson's steamer. Hearson's steamer (fig. 5) consists of two copper 

 cylinders, one suspended within the other, thus conserving the heat. By 

 means of a special regulator the gas is automatically lowered when the inner 

 chamber is full of steam, and this, instead of escaping into the sterilizing 



FIG. 4. Koch's steamer. 



