152 APPLIED BACTERIOLOGY FOR NURSES 



destroy all disease germs. Some culture-media, when 

 heated to that degree for so long a time, undergo un- 

 desirable chemic changes; such media are subjected 

 to what is known as fractional sterilization. They are 

 heated in the Arnold sterilizer for twenty minutes on 

 three consecutive days; the first heating destroys most, 



Fig. 46. Arnold's steam sterilizer (Boston Board of Health form). 



if not all, of the vegetating bacteria, and in the interval 

 between the first and second heating the spores possibly 

 present will develop into vegetative bacteria, in which 

 state they are easily killed by the second heating. The 

 process is repeated on the third day, in order to insure 

 the death of all organisms which may have escaped the 

 first two heatings. 



