TUBE METHODS 153 



24), when packed with ice, or on the metal cooler, shown in 

 Fig. 26, through which cold water circulates. The plates 

 are labeled to correspond with their respective dilutions 

 and are then set aside, protected from dust and light until 

 colony development begins. In the case of gelatin the 

 plates must not be maintained at a temperature higher 

 than that of an ordinary living room, about 20 to 22 C. being 

 the most favorable. In the case of agar-agar the plates may 

 be maintained at the temperature of the animal body, i. e., 

 between 37 and 38 C. 



TUBE METHODS. 



Esmarch Tubes. A useful modification of the plating 

 method just described is that suggested by von Esmarch. 

 It insures the greatest security from contamination by 

 extraneous organisms and requires the least amount of 

 apparatus. It differs from the other methods thus: the 

 dilutions having been prepared in tubes contain a smaller 

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

 5 to 6 c.c. are, instead of being poured upon plates or into 

 dishes, spread over the inner surf ace of the tubes containing 

 them, and, without removing the cotton plugs, solidified 

 in this position. The tubes then present a thin cylindrical 

 lining of gelatin or agar-agar, upon which the colonies 

 develop. In all other respects the conditions for the growth 

 of the organisms are the same as in flat plates. 



The solidification of the media on the inner sides of the 

 tubes is best accomplished by rolling them upon a block 

 of ice (Fig. 27), after the plan devised by Booker in 1887 in 

 the Pathological Laboratory of the Johns Hopkins Univer- 

 sity. In this method a small block of ice only is needed. 



