THE MICROBIOLOGY OF THE ATMOSPHERE 



in field work, Miquel studied the microbial population of the air at high 

 altitudes in the Alps by volumetric methods (1884, p. 524); with the help 

 of a sea captain, M. Moreau, the air over the sea was studied on voyages to 

 Rio de Janeiro, Odessa, Alexandria, and La Plata; the micro-organisms 

 brought down in rain-water were caught, precipitated, and counted; 

 hourly variations of fungus spores and bacteria in the air were studied 

 on improved volumetric traps with sticky slides, or on paper impregnated 

 with nutrient media and moved by clockwork. At Montsouris, fungus 

 spores showed a diurnal periodicity with tw^o maxima at about 8 and 20 

 hours, regardless of wind velocity. When he pressed the study of changes 

 in spore content of the air with passage of time still further, Miquel found 

 that the hourly reading was merely a smoothing of still shorter-term 

 variations. 



Trapping airborne bacteria at Montsouris on a moving paper disc 

 imbibed with nutrient agar, Miquel (1885) observed a regular diurnal 

 periodicity — with two maxima at approximately 7 and 19 hours averaging 

 about 750 per cubic metre, and with two minima at approximately 2 and 

 14 hours averaging about 150 per cubic metre. This periodicity was not 

 related to wind direction, and was not altered by moderate falls of rain. 

 In the centre of Paris the bacterial content also showed two maxima and 

 two minima, but there the minima were about equal to the maxima at 

 Montsouris, and the times of the maxima were closely related to activities 

 in the city such as sweeping the street, and to the passage of horse-drawn 

 traffic . 



Miquel appears to have been overwhelmed by the richness of the 

 information on the mould spore flora provided by his apparatus, for he 

 promptly abandoned it, merely remarking 'the micrographer ^^ ho has the 

 leisure could make some nice [curieuse] studies of this subject'. It was, 

 however, not abandoned before the main elements in the mould-spora had 

 been discovered by this excellent method. 



Interest in the mould-spora waned when it became clear that the 

 devastating epidemic diseases prevalent from time to time in cities were 

 not fungal in origin but were due to bacteria, and attention became ur- 

 gently focused on drinking water as the source of many of the current 

 epidemic fevers abounding in Paris. The laboratory at Montsouris then 

 became the centre for the bacterial analysis of samples of drinking water 

 sent from wells in Paris and other parts of France. 



Meanwhile, in Germany, the work of W. Hesse {b. 1846, d. 191 1) had 

 proceeded along similar lines. Hesse's apparatus for air sampling con- 

 sisted of a narrow horizontal tube, 70 cm. long and 3-5 cm. wide, con- 

 taining a layer of Koch's nutrient gelatine. A known volume of air was 

 aspirated slowly through the tube, and micro-organisms settled and grew 

 on the medium. Most colonies developed near the entrance to the tube, 

 and Hesse assumed that by the time the slow stream of air had reached 

 the end of its 70 cm. course all micro-organisms had been precipitated 



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