THE MICROBIOLOGY OF THE ATMOSPHERE 



weather coloured spores abounded. Re-invasion of the air after rain was 

 mostly by colourless organisms, which Miquel diagnosed, probably 

 incorrectly, as immature spores. Rain and warmth increased the atmos- 

 pheric fungus spore content, though this might decrease during high 

 winds because their extra lifting power did not compensate for their power 

 of desiccation and, hence, killing. 



DIURNAL PERIODICITY 



(i) Diurnal periodicity in numbers of bacteria outdoors could be studied 

 only on selected days of the year because, by Miquel's methods, for 

 hourly studies from 600 to 700 culture flasks had to be handled in one 

 day. The fullest data are probably those of 1884, which showed continual 

 change from hour to hour of the day in relation to changes in meteorological 

 factors that have not yet been unravelled. 



NOON 



Fig. 19. — Diurnal periodicity of total numbers of bacteria in air at the Observatoire 

 Montsouris, Paris, based on hourly readings between March 1882 and September 1884 

 (Miquel, 1886). 



Bacterial numbers showed diurnal periodicities differing between 

 Montsouris and the centre of Paris. At Montsouris there were two daily 

 maxima at o8-oo hours and 20-00 hours, and two minima at 02-00 hours and 

 14-00 hours, respectively (Fig. 19). In the centre of Paris, however, there 

 tended to be a single maximum at 14-00 hours and a minimum at 02-00 

 hours during most of the year, but in autumn the double peaks tended to 

 occur in central Paris as at Montsouris. 



This diurnal variation was shown to hold irrespective of wind direc- 

 tion — an effect that was possibly in part attributable to mechanical causes 

 such as traffic and the sweeping of streets. Furthermore the peaks occurred 



112 



