126 GENERAL BACTERIOLOGY 



CHAPTER V 

 BACTERIOLOGICAL ANALYSIS 



EXERCISE 66. COMPARATIVE ANALYSIS OF AIR (Koch). 



a. Plate three tubes of gelatin and expose by removing lid for 

 20 minutes in the following places: 1. Laboratory. 2. Cellar. 

 3. Out of doors. 



h. Replace the lids and keep plates at 22° C. for several days. 



c. Count the colonies. The counting is facilitated by the use of 

 Plate II. on which the Petri dishes are to be placed. In counting 

 a hand lens magnifying about 5 diameters should be used. Where 

 possible all of the colonies on the plate should be counted, if this 

 be impossible count a representative area and estimate the whole 

 number. 



d. Express the results in terms of the number of organisms 

 which fall per square foot per minute. The area of the Petri dish 

 can be read off directly from Plate II. in square centimeters, or it 

 can be calculated by multiplying the square of the diameter by 

 0.785. 



This method enables one to make a rough comparison of the 

 number of organisms occurring in the localities examined, but to 

 determine the number per voluuie the following method must be 

 employed. 



Reference. H. 477. 



EXERCISE 67. QUANTITATIVE DETERMINATION OF NUMBER OF 

 BACTERIA IN AIR (Petki-Sedgwicic). 



General Directions. 



a. A piece of glass tubing 6 mm. (14 in-) in diameter by 15 cm. 

 (6 in.) long is drawn out at one end in a gas flame and sealed. 



&. Fill this tube about one-third full with granulated sugar, 

 insert a cotton plug next to the sugar, and one at the end of the tube 

 (Fig. 25, A). 



