DUST AND AIR POLLUTION 77 



and the like do not occur in dust-free air, and are due to condensation 

 of moisture upon dust particles. And much the same has been 

 found to be true in respect to dust and bacterial pollution. As a 

 rule, when the former is abundant the latter is considerable. Haldane 

 and Osborn (vide infra) found bacteria most numerous in a workplace 

 where dust was most abundant, and their finding was merely con- 

 firmatory of many other previous researches. On the other hand 

 it should be remembered that, though dust forms a vehicle for 

 bacteria, dusty air is sometimes comparatively free from bacteria. 



For the conditions which affect the number of bacteria in the 

 air are various. In open fields, free from habitations, there are 

 fewer, as would be expected, than in the vicinity of manufactures, 

 houses, or towns. A dry, sandy soil or a dry surface of any kind 

 will obviously favour the presence of organisms in the air. Frank- 

 land found that fewer germs were present in the air in winter than in 

 summer, and that when the earth was covered with snow the number 

 was greatly reduced. Miquel and Freudenreioh have declared that 

 the number of atmospheric bacteria is greater in the morning and 

 evening between the hours of six and eight than during the rest of 

 the day. 



There is no numerical standard for bacteria in the air as there is 

 in water. In houses and towns it would rise according to circum- 

 stances, and frequently in dry weather reach thousands per cubic 

 metre. When it is remembered that air possesses no pabulum for 

 bacteria, as do water and milk, it will be understood that bacteria 

 do not live in the air. The quality and quantity of air organisms 

 depend entirely upon environment and physical conditions. In some 

 researches which the writer made into the air of workshops in Soho 

 in 1896, it was instructive to observe that fewer bacteria were isolated 

 by Sedgwick's sugar-tube in premises which appeared to the naked 

 eye polluted in a larger degree than in other premises apparently less 

 contaminated. In the workroom of a certain skin-curer the air was 

 densely impregnated with dust particles from the skin, yet scarcely 

 a single bacterium was isolated. Macfadyen and Lunt have also 

 found that the number of dust particles does not bear any relation to 

 the number of bacteria. They found that air containing even 

 millions of dust particles might be almost germ-free. In the polish- 

 ing room of a weU-known hat firm, in which the air appeared to the 

 naked eye to be pure, and in which there was ample ventilation, 

 there were found by the writer numerous bacteria belonging to four 

 or five species of saprophytes. The public analyst, for the city of 

 Nottingham, estimated the bacterial quality of the air of the streets 

 of that town during "the goose fair" held in the autumn. He 

 used a modification of Hesse's apparatus, in which the gelatine is 

 replaced by glycerine. The air was slowly drawn through and 



