98 BACTERIOLOGICAL LABORATORY TECHNIC 



plished by heating the media to 100 C. on three successive days 

 in a Koch or Arnold sterilizer. By heating culture media to this 

 temperature, all the vegetative, or adult, forms are killed, while the 

 spores are not affected ; after the first sterilization, at room tempera- 

 ture, the spores vegetate and become adult bacteria, when on the 

 second sterilization they are non-resistant to 100 C. and are killed. 



FIG. 24. Arnold sterilizer. 



Spores remaining after this develop into adult forms again 

 and are killed on the third day, at the third sterilization. This 

 fractional sterilization is employed in many laboratories still, and 

 is certainly the best for media containing carbohydrates of any 

 kind. To be effective, the media must be exposed to a temperature 

 of 100 C. for thirty minutes, that is, thirty minutes after the steam 

 has begun to form. Over heating of sugars causes them to 

 caramelize and turn black. 



