STERILIZATION OF GLASSWARE 27 



about 23 pounds to the square inch is necessary, that is, 8 pounds 

 more than the ordinary atmospheric pressure of 15 pounds. To 

 reach 120 C. the pressure must mount to 30 pounds or 15 pounds 

 plus the atmospheric pressure. Temperature and pressure corre- 

 spond thus: 



Temperature about 115 C. Increased pressure necessary, 8 Ib. 

 Temperature about 120 C. Increased pressure necessary, 15 Ib. 



A temperature of 120 C. maintained for 15 minutes is sufficient 

 to sterilize media in test tubes. Media in bulk is generally allowed 

 30 minutes. 



In using the autoclave care should be taken (1) that baskets of 

 tubes are not placed one on the top of the other or the plugs will 

 become wet from the dripping. (2) All air should be displaced by 

 steam before closing the stopcock. If a mixture of air and steam 

 is present the exact temperature is not recorded. (3) The pressure 

 should be allowed to drop to zero before opening the stopcock; 

 a too rapid removal of the pressure may cause the fluid media to 

 be blown out of the tubes. 



The autoclave is used mainly for the sterilization of sugar- 

 free media and discarded cultures. As gelatin, blood serum, 

 and all media containing sugars are changed chemically when 

 heated for a long time at a high temperature, they are sterilized 

 by another method. 



Discontinuous Sterilization, sometimes spoken of as intermittent 

 or fractional sterilization. There are two forms of applying this 

 method : 



1. Heating in steam at 100 C. for 20 minutes on each of six successive 



days. 



2. Heating in steam from 56 C. to 70 C., 1 hour on each of three suc- 



cessive days. 



The former method is used for gelatin and all sugar media that 

 would be injured by autoclave sterilization; the latter method 

 is employed for media containing blood serum or transudates 

 from body cavities, such as ascitic fluid. 



The principle underlying both methods is : all bacteria when 

 free from spores are killed by the temperature of boiling water, 



