20 CULTURE METHODS AND TECHNIQUE 



of from fifteen to twenty minutes at this temperature will usually 

 sterilize any medium. The period of incubation must of course be 

 measured from the time the desired temperature is attained, and it 

 may require from ten to fifteen minutes, even with a strong burner 

 system, in order to reach this temperature. An autoclave contain- 

 ing an ordinary amount of culture vessels should, if provided with 

 a double ring of burners, and jacket, develop a pressure of 15 lbs. 

 in about ten minutes. 



Temperatures higher than 1 1 5° may transform, possibly through 

 acidity, many sugar-containing and other organic media, and con- 

 sequently greatly reducing their value for the growth of many 

 organisms. Gelatin and milk are injured, if acid, by autoclave tem- 

 peratures. With more readily decomposable substances sometimes 

 necessarily employed in phytopathological work, it may be possible 

 to effect sterilization at temperatures below the boiling point, at 

 from 80° to 90° C, say, sterilization being made on about six suc- 

 cessive days. The blood serum incubator may also be employed. 



Not only does the autoclave facilitate sterilization, but it is eco- 

 nomical in the preparation of media to such an extent that it should 

 be in general use. The expense of such a piece of apparatus is the 

 one factor operating against its general adoption, yet a good instru- 

 ment handled carefully should last a number of years. 



The autoclave may be heated by burners, or it may be connected 

 with a steam supply pipe, if a supply of steam under sufficient 

 pressure may be constantly at hand. Autoclaves are usually pre- 

 pared for gas burners. In using this instrument care should be 

 taken to arrange mechanical reminders if there is danger of its 

 being neglected even for half an hour. It might be suggested that 

 an alarm clock as such is useful, or that a clock arrangement for 

 shutting off the gas at the desired time is in use in some labora- 

 tories. With the gas-heated autoclaves, particularly, certain precau- 

 tions are necessary. Before each sterilization it must be noted that 

 sufficient water is present, usually up to the crate or false bottom ; 

 and it is well to employ distilled water. The lid and other fittings 

 should be kept free of dirt and dust, so that all fastenings may be 

 tight and secure. If the burner capacity is not too great, the gauge 

 screw may with little practice be set at the temperature desired, the 

 steam vent left open, and the apparatus lighted. A few minutes after 



