266 BACTERIOLOGICAL EXAMINATIONS 



In the interior of closed tanks and in the depths of sand niters 

 anaerobic conditions prevail. On beds of coke, and on the surface 

 of sand niters, aerobic conditions obtain. The effluent from a 

 septic tank sewage disposal plant is very often pure water from 

 both chemical and bacteriological standpoints, due to the chemical 

 action of the bacteria. 



Bacteriology of the Air. 



That the lower layers of the earth's atmosphere contain many 

 bacteria is well known. The air over the sea and over mountain 

 ranges is freer from bacteria than is the air over arable lands and 

 large cities. 



When air is still and confined, all bacteria, according to Tyndall, 

 gravitate to the ground, and the air above becomes quite sterile. 

 The atmosphere of sick rooms, hospitals, public conveyances, 

 theatres, etc., contains many bacteria and often pathogenic ones. 



The pus cocci, tubercle bacilli, and the organisms causing small- 

 pox, scarlet fever, and measles, all may contaminate the air. 



The number of bacteria in a given quantity of air may be accu- 

 rately measured by means of a Sedgwick-Tucker aerobioscope ; this 

 consists of a large cylindrical glass vessel opening at either end into 

 various tubulations. (Fig. 85.) Into one of these granulated 

 sugar may be packed; the ends are then plugged with cotton and the 

 apparatus sterilized. To examine the air, a litre or more is drawn 

 through the sugar and the latter is then shaken into the large 

 cylinder where it is dissolved in melted gelatine culture media. 

 The latter is distributed over the interior of the glass and allowed 

 to harden. All the bacteria that were in a litre of air having been 

 mixed with gelatine and those that are not strict anaerobes grow in 

 the gelatine and a number of colonies can then be counted. 



The dust of dwellings and streets contains most of the bacteria. 

 Dried sputum is ground under foot and swept up in gusts of wind, 

 and the contained bacteria are thus inhaled and do harm. The 



