GENERAL CHARACTERS OF BACTERIA Ig 
accomplish the purpose. Even boiling is not sufficient to destroy 
spores, so, to be sure of complete sterilization, a temperature 
above boiling is necessary. If the object is a solid that can bear 
heat it is simply heated at about 300°F. for an hour or so. If it 
is a liquid it is placed in an autoclave (Fig. 12), and heat is applied 
until there is a steam pressure from 
ro to 15 pounds. This produces a 
temperature sufficient to destroy 
spores. 
Sometimes it is desirable to ster- 
ilize liquids that will not stand these 
high temperatures. This can be 
accomplished by discontinuous heat. 
The material is heated to about 
180°F., or more commonly to boiling. 
It is then cooled and allowed to 
stand twenty-four hours in a warm 
place. The heat has destroyed the 
active bacteria, but has not killed 
the spores, which, during the twenty- 
four hours, will germinate and grow 
into active bacteria. Heat is again 
applied as before, and this time any ic, 12—An autoclave used 
active bacteria that may have come 39 Sterilization of liquids under 
pressure (Eyr). 
from the germination of the spores are 
killed. The material is allowed to stand another day so that any 
spores that may have failed to germinate the first day may grow, 
then heat is applied again. Experience shows that three heatings 
of this sort will destroy all the organisms and sterilize the liquid. 
To be successful in this method it is necessary that the interval 
between the heatings should be long enough for the spores to 
germinate, but not long enough for the bacteria arising from them 
to form any more spores. Twenty-four hour intervals have been 
found to be the best. 
