EXAMINATION OF AIR, SOIL, AND WATER 243 



fifty colonies; on the agar plate at 37 C. ten colonies, five of 

 which were acid-formers, or presumably Bacillus coli. 



To count the colonies which develop upon the plates, a 

 special apparatus has been designed, known as 



WolfhilgeVs Counter. A glass plate divided into squares, 

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

 plate is placed above the dish with the colonies, and the num- 

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

 smaller ones. 



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



Bacterial Treatment of Sewage. Where sewage is to be 

 rendered innocuous before being allowed to flow into streams, 

 the process of nature has been imitated by the construction 

 of septic tanks in which the sewage remains excluded from 

 the air and subject to the action of the anaerobic bacteria 

 present in the sewage. The organic nitrogen is reduced, and 

 compounds of hydrogen and sulphur are formed. The 

 effluent is then filtered through coke-beds, where the aerobic 

 bacteria assist in further purification and over sand filters, or 

 exposed to the air on contact beds. No method of sewage 

 purification is very practical or safe. Pure water should not 

 depend on the efficiency of sewage filtration, but should be 

 obtained from a reasonably pure source. 



Sewage is also treated by sedimentation with alum and 

 filtration of the effluent over larger beds, or allowed to per- 

 colate through the soil, which is thereby enriched and utilized 

 for agriculture. It is also dried and sold in a compressed 

 form for fertilizer. 



The Examination of the Soil. The upper layers of the 

 soil contain a great many bacteria, but because of the diffi- 

 culty in analyzing the same, the results are neither accurate 

 nor constant. 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 embedded in the gelatin, 

 and may contain within 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. 



