26 



LABORATORY EXERCISES IN BACTERIOLOGY. 



C. ; that of partially confined steam (as in any of the common steam sterilizers, where the 

 lid of the receptacle is closed, but not tightly clamped, and where perhaps there is in ad- 

 dition some opening in the lid also serving as a means of moderate escape of the vapor) 

 is about 100 C. or that of the boiling water; while the temperature of steam confined 

 under pressure, as in the specially constructed steam boilers known as steam digesters or 

 autoclaves, may be raised considerably above that of boiling water. The exact degree 

 in the latter case varies with the degree of confinement, whether one or more atmos- 

 pheric pressures above normal, and with the amount of water in the boiler (the less water 

 after development of the steam, the higher the temperature for a given application of 

 heat). 



Steam as a sterilizing agent is of value because of its penetrative power as com- 

 pared with dry heat, the evenness of its contact, 

 and steadiness of its action. Exposure to steam 

 at 100 C. may be reckoned as of equal efficacy 

 for sterilization as dry heat at 150C. ; and 

 ordinarily it is the custom to allow the same 

 period of exposure as that employed in the use 

 of dry heat, twenty to thirty minutes, at inter- 

 vals of twenty-four hours, upon three successive 

 days. If the substance to be sterilized is not 

 liable to injury upon prolonged exposure, it 

 may be left in the steam at 100 C. for an 

 hour with a fair certainty that all adult bac- 

 teria and spores will at the end of that time be 

 destroyed. For rapid and complete steriliza- 

 tion, however, exposure to high temperature in 

 steam under pressure in an autoclave is usually 

 practised, a temperature of 120 C. and pres- 

 sure of thirty pounds to the square inch being 

 usually employed, an exposure of from fifteen 

 to twenty minutes being generally lethal to 

 both adult bacteria and their spores. 



(a) In ordinary steam sterilization some 

 one or other form of steam boiler is made use 

 of, in which is attained a temperature of 100 C., 

 or under special arrangement somewhat less. 

 Perhaps the simplest type, and that most con- 

 venient for a variety of purposes, is the Koch pattern (Fig. 5), consisting of a rather 

 tall, heavily tinned sheet-iron boiler, with copper bottom, arranged upon an ordinary 

 iron tripod or upon an inclosed iron stand, covered with asbestos felt to prevent the 

 rapid radiation of heat, and fitted with a convenient water gauge and a moderately 

 close fitting cover in which a suitable perforation is often provided for the accommoda- 

 tion of a thermometer. In the interior above the water-level is arranged a false 

 bottom of perforated metal. When in use, water is put in the boiler so as to cover the 

 bottom for four or five inches, but not enough to reach the false bottom ; heat is applied 

 by a convenient burner and the water brought to the boiling-point. The articles to be 

 sterilized are placed in the steam chamber, resting upon the false bottom, and the 

 cover applied. There is no need of watching the temperature, and there is therefore no 

 occasion for a thermometer, since the temperature cannot rise above 100 C. with no 



FIG. 5. KOCH STEAM STERILIZER. 



