1 8 THE GENERAL CHARACTERS OF MICROORGANISMS. 
liquid it is placed in an autoclave (Fig. 12), and heat is applied until 
there is a steam pressure from 10 to 15 pounds. This produces a 
temperature sufficient to destroy spores. 
Sometimes it is desirable to sterilize liquids that will not stand 
these high temperatures, as, for instance, gelatin (Experiment No. 
1 2) . 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 active bacteria that may have come 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. 
Relation to Cold.- While heat will destroy all bacteria, cold will 
not do so. It is practically impossible to destroy the life of bacteria 
by freezing, at least with any certainty; for no matter how low the 
degree of temperature used, the life of some of these organisms seems 
to be totally resistant. Experiments have shown that bacteria 
cooled to the temperature of liquid air, or even liquid hydrogen, are 
not all killed, but after being warmed are still able to germinate. 
Although these extremes of temperature, do not destroy all bacteria, 
the simple matter of freezing and thawing will kill a great number 
of them. If water containing a large number of bacteria is frozen 
and subsequently thawed, the bacteria will be found much reduced 
in numbers, although they are not by any means all killed. When, 
therefore, water is contaminated by sewage containing typhoid 
bacteria, and ice is collected from it for domestic purposes, the 
typhoid bacteria may still be found alive in the ice. Such ice may 
