66 



BACTERIOLOGY 



ilization, heating for a short time on several consecutive days. 

 The temperature employed varies according to the effect desired. 

 A temperature of 60 C., maintained throughout a watery liquid 

 for twenty minutes will kill most vegetative bacteria, and practi- 

 cally all pathogenic bacteria which do not form spores. Such 

 partial sterilization is called Pasteurization. Boiling water, 100 

 C., kills vegetative bacteria in a very short time, less than two min- 

 utes for most bacteria, and the 

 spores of many species are de- 

 stroyed by boiling for 5 to 30 min- 

 utes. Some spores, however, for 

 example those of some varieties of 

 B. vulgatus, may survive a boiling 

 temperature for several hours. 

 Boiling is one of the most useful 

 practical methods of disinfection. 

 Nearly all pathogenic bacteria are 

 quickly killed in boiling water. 

 Surgical instruments are com- 

 monly boiled in water to which 

 sodium carbonate, i to 2 per cent, 

 has been added. Rusting and 

 corrosion may also be prevented 

 by adding 10 per cent of borax 

 to the water in which metal in- 

 struments are boiled. Steriliza- 

 tion of bacteriological media is 



usually done by means of streaming steam, rather than by immer- 

 sion in boiling water. The Koch steam sterilizer is a compara- 

 tively simple device for this kind of sterilization. It is a tall, 

 cylindrical, tin vessel covered with asbestos or felt. The lower 

 portion is filled with water; on the side is a water-gauge indicat- 

 ing the height of the water, in order .that one may observe when 

 there is danger of the sterilizer boiling dry. Over the top there 

 is a tight-fitting cover. The steam is generated by a Bunsen 



FIG. 29. Koch's steam sterilizer. 



