ESMAROH'S TUBES. 



107 



method, and at the same time lessens very materially the 

 number of steps in the process, is that suggested by Petri. 

 It consists in substituting for the plates small, round, 

 double glass dishes, which have about the same surface- 

 area as the plates. The liquid medium may be poured 

 directly into these little dishes without their being exactly 

 level. Each dish acts as a plate. Their covers are then 

 to be replaced, and they are set aside for observation. 

 In all other respects the steps are the same as those given 

 for Koch's original method. Petri's dishes are flat, double 

 dishes of glass (Fig. 24). They are of about 8 cm. in 



Fig. 24. 



Petri double dish, now generally used Instead of plates. 



diameter and about 1.5 to 2 cm. in height, the walls 

 being vertical. They may readily be sterilized either 

 by the hot-air or steam methods of sterilization. They 

 are very useful for this work, as they do away with the 

 necessity for the cooling-stage and levelling tripods, 

 though in warm weather the cooling-stage may be used 

 to hasten the solidification of gelatin. 



Esmarch's Tubes. — The modification of Koch's 

 method which insures the greatest security from con- 

 tamination by outside organisms and requires the small- 

 est supply of apparatus, is that suggested by v. Esmarch. 

 It differs from the other methods thus : The dilutions 

 having been prepared in tubes containing a smaller 

 amount of medium than usual — as a rule not more than 



