ISOLATION AND PURE-CULTURE METHODS 21 



steam begins to escape vigorously, or practically as soon as the 

 thermometer registers ioo° C, the vent is closed. It is not advis- 

 able to leave the autoclave without observation during sterilization, 

 since there are many chances for mishaps ; nevertheless, if the 

 safety valve is set for a given pressure, steam will, of course, be 

 blown off at about the temperature desired. This blowing off of 

 steam is a good signal for cutting off a part of the gas supply, as 

 the rapid escape of steam not only results in exhaustion of the small 

 reservoir, but often dislocates the cotton plugs. When the time 

 for sterilization has elapsed, the gas is turned off ; but the steam 

 vent should be only gradually opened as the pressure falls to ioo° C, 

 else the medium may boil over and the plugs will be blown out of 

 the vessels. If steam is used instead of a gas burner, a complicated 

 set of stop-cocks will be required, or will at least be advantageous, 

 to regulate the inlet and exit of the steam. 



Hot air sterilization. Implements, glassware, cotton, sand, and 

 other vessels or materials used in culture work, which may not be 

 sterilized by steam or by the burner flame direct, are sterilized in 

 a hot air sterilizer. It is true that the delicate mycelium and spores 

 of many fungi are often injured or killed by drying alone ; yet, on 

 the other hand, the spores and mycelium of many fungi are ex- 

 tremely resistant to desiccation and to a high degree of dry heat. 

 By long practice it has been ascertained that it is not safe to attempt 

 to sterilize vessels in a dry oven at less than 150 C. for one hour, 

 or at a slightly lower temperature with protracted sterilization. Test 

 tubes or flasks plugged with cotton, or Petri dishes wrapped with 

 paper, cannot well be exposed to a much higher temperature. 

 Glassware may safely be exposed to a temperature of 170° C, or 

 higher. The best form of hot air sterilizer is the Lautenschlager, 

 Fig. 3, yet a simple and inexpensive oven will suffice. 



Sterilization of soil. In all inoculation work where there is 

 danger of contamination from the soil, and particularly in the study 

 of root and stem diseases, experiments with damping-off fungi, and 

 the like, it is necessary to use sterile soil and sterile pots. The 

 pots may be prepared with soil as for the growing of any plants, 

 well watered, and then sterilized a few at a time in any steam ster- 

 ilizer or autoclave. In the former they should be sterilized at least 

 two or three hours after the temperature has reached 100 C, and 



