CHAPTER V. 
BACTERIOLOGICAL ANALYSIS. 
EXERCISE LXV. COMPARATIVE ANALYSIS OF AIR (Koch’s [ethod). 
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. 
b. Replace the lids and keep plates at 22° C. for several days. 
e. Count the colonies; if the number of colonies is greater than 100, use the 
counting plate figured in Plate I. A. and count a portion and estimate the whole number. 
d. Calculate the area of the Petri dish by multiplying the square of the diameter 
by 0.785. 
e. Express the results in terms of the number of organisms which fall per 
square foot per minute. 
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 volume the fol- 
lowing method must be employed. 
REFERENCES. H. 390. 
EXERCISE LXVI. QUANTITATIVE DETERMINATION OF NUMBER OF BACTERIA IN AIR 
(Petri-Sedgwick Method). 
GENERAL DIRECTIONS. 
g a. A piece of glass tubing 6 mm. (2 in.) 
Hf in diameter by 15 em. (6 in.) long is drawn out 
at one end ina gas flame and sealed. 
b. Fill this tube about one-third full : 
with granulated sugar, insert a cotton plug next a;:-, 
to the sugar and one at the end of the tube 
(Fig.13). 
c. Sterilize in the hot air sterilizer for A 
1 and % hours at 130° C. (sugar melts at a 
higher temperature) . . » ey 
d. Fasten the tube, pointed end up, in ~ 
a clamp, remove the first cotton plug and con- 
-nect with an aspirator. (Fig. 14). 
Fic.14. Aspirator for é. Break off the pointed end of the tube 
filtering air. and draw a measured quantity of air through Fic.13. Apparatus for fil- 
the sugar. tering air through sugar. 
A, ready for sterilization. 
SPECIAL DIRECTIONS. B, point broken off and at- 
a. Filter 50 liters of air. taghed ty aspirator, 
b. Dissolve sugar in 10 ce. of sterile water and make plates, using 1 ec. of the 
mixture. : 
c. Incubate, count colonies as above and estimate the number of organisms per 
liter of air. ; 
REFERENCES. A. 551; H. 393; L. & K. 392; McF. 164; N. 449; §. 541. 
