90 METHODS OF CULTIVATION OF BACTERIA. 



to see that there is not too much evaporation from the 

 surface of cultures placed within incubators, otherwise they 

 may quickly dry up. It is thus advisable to raise the 

 amount of water vapour in the interior by having in the 

 bottom of the incubator a flat dish full of water from which 

 evaporation may take place. Tubes which will require to 

 be long in the incubator should have their plugs covered 

 either by india-rubber caps or by pieces of sheet rubber tied 

 over them. These caps should be previously sterilised in 



FIG. 37. Hearson's incubator for use at 37 C. 



i- 1 ooo corrosive sublimate and then dried. Before they 

 are placed on the tubes the cotton-wool plug ought to be 

 well singed in a flame. " Cool " incubators are often used 

 for incubating gelatine at 21 to 22 C. Here again 

 Hearson's design is as good as any in the market. 



Permanent Preservation of Cultures. This may be 

 conveniently effected by means of formalin vapour. The 

 cotton-wool of the tube containing the culture to be 

 preserved is removed and soaked in formalin (40 per cent 

 formol aldehyde). It is then replaced and covered with an 



