70 APPLIED BACTEEIOLOGY 



soon as they are quite solidified they are set aside for the 

 colonies to develop. These tubes possess the advantage 

 that they are quickly made, they do not occupy much 

 room, and are well protected against atmospheric germs. 



Fig. 16. — Method of making Esmaroh's Eoll Cuiturb. 



When the colonies have formed, they can readily be 

 counted and examined by means of a lens. 



Agar Plates. — The characters of the growth on nutrient 

 agar media are not so varied as in the case of those on 

 gelatine, and the plates are rather more difficult to manage; 

 but this medium possesses the advantage of not liquefying 

 at 40° C, whereby the nature of the growth can be studied at 

 higher temperatures than is possible in the case of gelatine. 



Agar media are not liquefied in the manner that gelatine 

 media are by many organisms. 



The tubes containing the nutrient agar are stood in a 

 beaker of boiling water until the contents are completely 

 melted. After this the water in the beaker is cooled 

 somewhat, and then allowed to stand, with a thermometer 

 immersed in the water until a temperature of 40° C. is 

 reached. The tubes are then immediately inoculated and 

 the contents poured into a plate, as previously directed 

 for the preparation of gelatine plates. It is a good plan 

 to warm the dishes or plates to 40° C. before pouring the 

 agar, and, above all, to work quickly, as the agar solidifies 



