BACTEEIA IN THE AIR 77 



difference will be observed in the two results. This was tried in a 

 classroom of the Dundee High School. A bacteriological examination 

 was made before and after the boys had been asked to stamp on the 

 floor. Before stamping, 11 bacteria, and after, 150 bacteria were found 

 per litre. We may, therefore, conclude that the atmosphere of towns 

 contains more bacteria than that of the country, that we inhale more 

 bacteria when on the highroad than when in the green fields, and more 

 when walking through a fog than when walking through a clear atmo- 

 sphere. The number of bacteria, however, does not always bear a 

 relation to the amount of dust in the atmosphere. Thus, the air in 

 the workroom of a certain skin-curer was found to be densely impreg- 

 nated with dust particles from the skins, but there was scarcely a 

 microbe present, while, on the other hand, the atmosphere of the 

 polishing-room of a hat firm was found to be remarkably free from 

 dust, yet several kinds of bacteria were isolated. 



Secondly, the number depends on the dampness of the surfaces. 

 The scarcity of bacteria in sewers has already been mentioned. The 

 same condition explains the fact that during quiet breathing no bacteria 

 are exhaled from the mouth, because the mouth and air passages 

 being moist, the bacteria are all captured. During speaking, sneezing, 

 and coughing, however, the case is different, for minute particles 

 containing bacteria are shot out of the mouth during these processes. 



Thirdly, the altitude of any particular place is an important factor, 

 for, as bacteria are subject to gravity, the higher up we go the fewer 

 bacteria do we find. This is clearly seen in the results of the investi- 

 gation into the condition of the atmosphere of the House of Commons. 

 At the top of the Clock Tower there were only TS bacteria and moulds 

 per litre, while halfway up this tower the number was 1'5. Twenty 

 feet from the ground the number rose to 3-3 per litre, and on the 

 ground itself 4'2 per litre were found. The same results have been 

 obtained in other investigations of this nature. Thus, on the spire of 

 Norwich Cathedral (310 feet) 10 litres of air yielded 7 micro organisms, 

 whereas on the tower (180 feet) 9 were obtained, and on the ground 

 18 for the same quantity of air. 



Fourthly, as bacteria are usually conveyed on particles of dust, we 

 have to take into account the influence of air-currents on the distri- 

 bution of bacteria. Other things being equal, there are more bacteria 

 in the atmosphere during a high wind than when the weather is calm, 

 owing to the amount of dust that the wind lifts from the ground. 



Finally, the specific nature of the soil which creates the dust exer- 

 cises a marked influence on the number of bacteria in the overlying 



