l8 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 



