82 General Bacteriology. 
EXERCISE LXVII. RELATION OF BACTERIA IN THE AIR TO DUST PARTICLES. 
a. Pour a tube of gelatin into a Petri dish and solidify. 
b. Remove the lid and shake a dust-brush or cloth over it. 
c. 18-24 hours later, examine under low power of microscope to determine the rela- 
tion of the developing colonies to the dust particles. 
EXERCISE LXVIII. ESTIMATION OF NUMBER OF BACTERIA IN SOIL. 
a. With a sterile knife collect a sample of soil in a sterile test-tube 
or Petri dish. Samples at various depths can be secured by means of an 
earth.borer. (Fig. 15). 
b. Weigh out 1 gram and dilute 1000 times with sterile water. 
c. Make three gelatin plate cultures using 1 cc., $cc. and zy ce. of 
this suspension. Incubate. 
d. Count the colonies as they develop and estimate the number of 
bacteria per gram of soil. 
e. Many of the bacteria of the soil are anaerobic and can only be 
grown in the absence of free oxygen. See Part II. Chap. VII. for methods of 
cultivation. 
REFERENCES. A. 556; H. 394; L. & K. 389; McF. 174; N. 444; S. 
567. 
Fig. 15. Fraenkel’s 
Soil Borer. 
EXERCISE LXIX. WATER ANALYSIS. 
; QUANTITATIVE ANALYSIS. 
a. Collect a sample of water in a sterile test-tube or bottle. Fig. 
16 shows a form of apparatus used in taking samples of water at vari- 
ous depths. 
b. Make two gelatin plates using $cc. and 7y cc. of the water. 
e. Count the colonies as they appear, and estimate the number 
per ce. 
‘d. Make agar plates and compare results with those obtained 
above. : 
e. Analyze asurface water (lake or river), a deep well and a 
spring water. 
QUALITATIVE ANALYSIS. 
a. Detection of putrefactive organisms. Examine gelatin plates, 
made above and (1) determine number of liquefying organisms per ce. 
(2) search for the presence of proteus forms. (B. vulgaris.) 
b. Detection of Faecal Bacteria. Pi Giatr Leip 
1) Inoculate a fermentation-tube containing glucose bouillon (1%) with 1 ce. 
of water. 
2) Make litmus lactose agar plate using 1 ce. water. 
3) Ineubate both at 38° C. 
4) Compare growth obtained with that of B. coli. 
REFERENCES. A. 526; H. 878; L. & K. 396; McF.169; M. & R. 79: N 
P. 245; S. 553. For the determination of the various Species present ace Bratt 
Micro-organisms of Water; Fuller: Report Am. Public Health Assoe., 1899, 580 
. 422; 
kland’s 
