HISTORICAL INTRODUCTION 



number of particles of various types contained in a measured volume of air. 

 Moulds were at first estimated microscopically in a 24-28 hour deposit, 

 obtained by impinging the air to be sampled on a glycerined glass slide 

 which was placed horizontally 2 to 3 mm. above a downward-facing 

 orifice. The diameter of the orifice was from 0-5 to 075 mm. Suction of 

 20 litres per hour was maintained by a water-operated pump (Miquel, 

 1879). Miquel found that this apparatus yielded about 100 times as 

 many particles as the aeroconiscopes designed by Maddox and Cunning- 

 ham, though for qualitative work away from the laboratory he still used 

 a wind-operated trap of the Maddox type. 



Bacteria, especially bacterial spores, could not be satisfactorily counted 

 microscopically and Miquel was forced to estimate them by cultural 

 methods. At first he drew known volumes of air through liquid media 

 (sterile beef extract, etc.), partitioning the liquid either before or after 

 exposure into 50 or 100 vessels, and adjusting the volume of air sampled 

 so as to leave from a quarter to a half of the vessels sterile — in order to get 

 a reliable estimate of the number of bacterial particles in the volume of 

 air sampled. The numbers of microbes in the air varied greatly in the 

 same place at different times, and this variation was studied in relation to 

 season, weather, district, and altitude. Miquel was the first to make a 

 long-term survey of the microbial content of the atmosphere by volu- 

 metric methods. 



In the Pare Montsouris, out-of-doors, Miquel estimated that the 

 mould spores averaged about 30,000 per cubic metre in summer, some- 

 times rising to 200,000 in rainy weather. In prolonged dry weather they 

 decreased in number, and were only about 1,000 per cubic metre in 

 winter, with very few indeed when snow was on the ground. While rain 

 was falling the numbers of mould spores usually decreased considerably, 

 but afterwards their numbers recovered quickly — in fact, much more 

 quickly than did those of particles of inorganic dust. Resting stages (eggs) 

 of infusoria were estimated at about i or 2 in 10 cubic metres of air. 

 Pollen grains in June may make up 5 per cent of the airborne organic 

 particles, while starch grains near habitations may account for i per cent. 

 Bacterial numbers out-of-doors in the Pare Montsouris were at first 

 estimated at about 100 per cubic metre; but improved culture media 

 increased this figure by a factor of 7 to 10 times. The numbers of bacteria 

 in the centre of Paris were, perhaps, 10 times as high again as in the Pare 

 Montsouris, with larger numbers inside dwellings, and still more in 

 crowded hospitals. The work showed signs of settling into a steady routine 

 with the publication of Miquel's Les organismes vivants de Vatmosphere^ 

 Paris, 1883. 



However, in 1883 and 1884 Miquel was stung into a burst of renewed 

 activity by the intrusion of a rival centre for the study of hygiene which 

 had been established in Berlin under W. Hesse, who used the new solid 

 media which Miquel abhorred. With the collaboration of de Freudenrich 



