BACTERIOLOGY OF THE AIR 283 



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. 82). Into one of these granu- 

 lated 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 



FIG. 82. Sedgwick-Tucker aerobioscope. (Williams.) 



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 

 air coming quietly from the lungs is pure and sterile. Even in 

 active disease processes of the throat this is true. In case the 

 breath comes violently, as in speaking, coughing, and sneezing, the 

 reverse is the case. In general it may be put down as an axiom 

 that disease germs cannot rise from a fluid, such as sewage. If 

 they could it would mean that they are lighter than air, which is 

 not the case. Sewer gas, as a rule, is a bearer of some pathogenic 

 bacteria chiefly cocci but in reality it is purer than generally sup- 

 posed. The spread of organisms from sewage only extends 3-6 

 metres into the atmosphere and then only the the bursting of bub- 



