156 APPENDIX. 



one-half the number on one plate that there is on the other ; 

 thus the ^ c.c. serves as control. 



Water that is very rich in germs requires dilution with ster- 

 ilized water 50 to 100 times. 



To count the colonies which develop upon the plates, a spe- 

 cial apparatus has been designed, for, unaided, the eye cannot 

 see them all. 



Wolf hugel's Apparatus. A glass plate divided into squares, 

 each a centimeter large, and some of these subdivided. This 

 plate is placed above the gelatine plate with the colonies, and 

 the number in several quadrants taken, a lens being used to see 

 the smaller ones. 



The petri saucers can be used instead of plates, and an appa- 

 ratus on the Wolfhiigel plan can be obtained to count the colo- 

 nies. It is best to count all the colonies on the plate or dish. 



Varieties Found. The usual kinds found are non-pathogenic, 

 but, as is well known, typhoid and cholera are principally spread 

 through drinking water, and many other germs may and do 

 find their way into the water. Many of the common varieties 

 give rise to fluorescence, or produce pigment. 



Eisenberg gives 100 different varieties as ordinarily found. 

 As mentioned before, 2 bacteria to a cubic centimeter, one of 

 them typhoid, give more danger to a water than thousands of" 

 non-pathogenic ones. When, however, more than 200 bacteria 

 to the c.c. are found, such a water ought not to be considered 

 potable. Distilled water forms often a good medium for some 

 bacteria. 



The Examination of the Soil. The upper layers of the soil 

 contain a great many bacteria, but because of the difficulty in 

 analyzing the same, the results are neither accurate nor con- 

 stant. The principal trouble lies in the mixing of the earth 

 with the nutrient medium ; little particles of ground will cling 

 to the walls of the tube, or be imbedded in the gelatine, and 

 may contain w.ithin them myriads of bacteria. As with water, 

 the soil must be examined immediately or very soon after it is 

 collected, the bacteria rapidly multiplying in it. 



When the deeper layers are to be examined, some precautions 

 must be taken to avoid contamination with the other portions of 

 the soil. One method, very laborious and not often practical, is 

 to dig a hole near the spot to be examined and take the earth 

 from the sides of this excavation. 



