STERILIZATION. 



59 



The sterilizer invented by Koch is still largely in use. It is 

 a tall, cylindical, tin vessel covered with asbestos or felt. The 

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

 indicating the height of the water, in order that one may ob- 

 serve when there is danger of the sterilizer boiling dry. Over 

 the top there is a tight-fitting cover. The steam is generated 



Fig. 14. — Koch's steam sterilizer. 



by a Bunsen burner standing underneath. A perforated shelf 



placed some distance above the surface of the water is for the 



reception of the tubes and flasks that are to be sterilized. 



The sterilization of blood-serum as a culture medium sometimes has to be 

 performed in a specially devised sterilizer, when a clear, fluid medium is desired. 

 In this case the serum is heated for an hour on each of six consecutive days to 

 a temperature of only 58° C. To obtain a transparent but solid medium the 

 serum is kept at a temperature of 75° C. for an hour on each of four consecutive 

 days. The process must be conducted carefully to avoid clouding of the 

 serum. 



