METHODS OF CULTURE 



47 



stance to be sterilized is placed in the steam and kept there 

 from ten to fifteen minutes, or longer, as required. 



Arnold's steam sterilizer (Fig. 16) will answer every purpose 

 of the Koch steam-chest. It is cheaper, also requiring less 

 fuel to keep it going. The steam does not escape, but is con- 

 densed in the outer chamber. 



The autoclave (Fig. 17), which produces steam under pres- 

 sure and allows a temperature of 120 C. to be obtained, is a 

 most effective method of steriliza- 

 tion, but the higher temperatures 

 are not suitable for gelatin or sugar 

 solution. Gelatin loses its power 

 of solidifying if the boiling is pro- 

 longed. 



Instead of sterilizing for a long 

 time at once, successive sterilization 

 is practised with nutrient media, 

 so that the albumin will not be 

 too strongly coagulated. Fifteen 

 minutes each day for three days 

 in succession in the steam-chest or 

 autoclave is sufficient. 



Fractional Sterilization of 

 Tyndall. Granted that so many 

 spores originally exist in the object 

 to be sterilized, it is subjected to 

 60 C. for four hours, in which time 

 a part at least of those spores have 

 developed into bacteria, and the 

 bacteria destroyed by the further 

 application of the heat. The next 

 day more bacteria will have formed, 

 and four hours' subjection to 60 C. 

 heat will destroy them, and so, at the end of a week, using 

 four hours' application each day, all the spores originally 

 present will have germinated and the bacteria be destroyed. 



As modified, and in use in most laboratories, fifteen minutes' 



Fig. 17. Modern autoclave. 



