64 



LABORATORY EXERCISES IN BACTERIOLOGY. 



for such rest, as shown in Fig. 13 ; however, there is added 'difficulty in cleaning this tube, 

 and it is therefore of questionable advantage. 



In the study of gas formation by bacteria, fermentation tubes of the same form as 

 those used in urinalysis for sugar fermentation and for the estimation of urea (Doremus' 

 ureometer) are usually used (Fig. 14). The collection arm of such tubes need not be 

 graduated, but the total capacity of the arm should be ascertained so that the amount 

 of gas may be approximately calculated in special cases. An efficient substitute for 

 such fermentation tubes may be extemporized by inverting a small test-tube filled with 

 the medium in a larger one, after the principle of a gas jar in a water-bath (Fig. 15). 



FIG. 18. PETRI DISH. 



2. Flasks. It is customary to employ flasks as containers for the nutrient media 

 in bulk, distribution to culture tubes being made from time to time as necessity de- 

 mands. It is best for this purpose to use the small sizes (as those of 250 to 500 cubic 

 centimeters capacity), since, should accidental contamination occur, smaller quantities 

 of the media are endangered. When large amounts of a culture of some special organ- 

 ism are desired, flasks are often used instead of tubes or dishes, and a number of special 

 forms of culture flasks may be obtained from the makers (Fig. 16). So, too, a number 

 of special forms of flasks have been devised for the storage of media and their ready and 



FIG. 19. PETRI DISH. 



safe distribution to tubes (Fig. 17). For general use, however, the ordinary flat-bot- 

 tomed Erlenmeyer flask is deservedly the most popular, its shape being favorable in 

 cleaning and its broad, flat bottom giving it stability. 



Flasks are to be cleaned, stoppered, and sterilized before use in precisely the same 

 manner as detailed for the culture tubes. Sand or shot agitated in the water will facili- 

 tate the cleansing of the interior, but it is best to use a long-handled brush obtainable 

 for the purpose. 



3. Dishes. Two types of dishes are in general use the large form used for potato 

 and plate cultures, and the small Petri dishes. 



