THE CULTIVATION OF MICRO-ORGANISMS 1 09 



should be exposed as little and as short a time as possible. Tubes 

 Nos. 2 and 3 are to be treated in the same manner. Burn the 

 plugs, and immerse the empty tubes in 5 per cent solution of 

 carbolic acid. Where much culture work is being done, it will be 

 found convenient to sterilize the mouth of each tube by thorough 

 heating in the flame after pouring out its contents, and then to 

 replace the plug. The tube may then be placed in a special 

 receptacle which is sterilized with its contents in the autoclave 

 at 120 C. for 20 minutes, at the end of the day's work. 



FIG. 41. Colonies in gelatine plate showing how they may be separated and the 



organisms isolated. 



The culture-medium in the Petri dish will soon solidify. 

 Petri dishes of agar should be inverted after the medium is firmly 

 set; otherwise the water, which evaporates from the surface and 

 condenses on the inside of the lid, may overflow the surface of 

 the agar, confusing the result. Agar plates are usually developed 

 in the incubator. Gelatin plates must be developed at a tempera- 

 ture below the melting-point of the medium, which is usually 

 between 22 and 28 C. Colonies usually appear in from one to 

 two days. In plate No. i they will be very numerous, in plate 



