62 PRACTICAL BACTERIOLOGY 



2. Take a plugged test-tube filled with warm water, lay it upon 

 the block of ice, and thereby melt a groove in the ice. 



3. The test-tubes (best filled with 5 to 6 c.c. of medium) are 

 placed in the groove after inoculation, and revolved rapidly with the 

 fingers of the right hand in one direction only. The left hand 

 holding the dish regulates the obliquity of the tube, which at the 

 beginning of the rotation should have the cotton plug at a higher 

 level. The media should not come in contact with the plug. 



This method can be used also for agar, but the tubes must be 

 kept slanted for about twenty-four hours, otherwise the agar will not 

 keep its position in the tube. After a certain number of hours the 

 agar in the vicinity of the cotton plugs dries and adheres to the glass ; 

 with gelatine no such difficulty occurs. Rubber caps are not 

 necessary. 



CXVI. METHOD OF COUNTING COLONIES IN 

 ROLL CULTURES. 



For counting colonies in roll cultures, an apparatus designed by 

 Esmarch (see Fig. 12) may be used ; it consists of a test-tube, holder, 



FIG. 12. Esmarch's Apparatus for Counting Roll Culture Colonies. 



and lens attached to a stand. The holder contains quadrangular 

 apertures of different sizes, the number of colonies being counted in 

 the larger or smaller apertures depending on the number of colonies in 

 the culture. A simpler method consists in attaching a piece of paper, 

 in which squares have been cut, to the tube by means of elastic bands, 

 and using an ordinary hand or watchmaker's lens (Nuttall). The 

 estimation of the total number is made, as with plate cultures, by 



