THE CULTIVATION OF BACTERIA. 87 



Petri dishes of agar should be inverted as soon as the medium 

 is thoroughly soUdified, otherwise the water which evaporates 



Fig. 31. — Dilution-cultures in Esmarch Roll-tubes. 



In tube I the colonies are very close together; in tube 2 they are somewhat 



separate; in tube 3 they are well isolated. 



from the surface condenses on the inside of the lid, and runs 



