THE CULTIVATION OF MICRO-ORGANISMS II3 



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. 



Pig. 43. — Colonies in gelatin 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 

 No. 2 less numerous, and in plate No. 3 still less numerous. 

 Where the number is small the colonies will be widely separated 



