164 MICROBES, FERMENTS, AND MOULDS. 
two or three days of damp, rainy weather, the rain- 
water often contains more bacteria than when it began 
to fall. Since the atmosphere is then excessively pure, 
it seems that the bacteria are able to live and 
multiply in the clouds, or else that the clouds, in 
their passage through space, take up a varying con- 
tingent of germs.” 
The maximum of air-germs is observed in autumn, 
the minimum in winter; thus, 50 bacteria were counted 
in December and January, only 33 in February, 105 
in May, 50 in June, and 170 in October. 
Inversely to what occurs with moulds, the number 
of bacteria, low in rainy weather, rises when all 
moisture has disappeared from the surface of the soil. 
The effect of dryness is greater than that of warmth. 
This explains the scarcity of bacteria after the great 
rains of February, April, and June. A long drought 
is, however, unfavourable to their development. 
Miquel’s experiments lead him to conclude that 
dew, the evaporation from the soil, is never charged 
with spores. The dry dust in the neighbourhood of 
inhabited places, and especially of hospitals, is, on the 
other hand, charged with microbes. In the centre of 
Paris, for example, in the Rue de Rivoli, there are 
nine or ten times as many microbes in the atmosphere 
as in the neighbourhood of the fortifications. In the 
Montsouris Observatory, south of Paris, the north 
winds bring many more bacteria than the south winds. 
The most impure wind comes from the hills of Villette 
