THE MICROBES OF HUMAN DISEASES, 161 
influence of rain and damp is very marked. In winter 
the average number of spores in every cubic metre of 
air does not exceed 7000, while in June it rises to 
35,000. 
In summer, however, when the temperature is 
very high, the number of spores is not great; for this 
reason, that, in spite of the heat, the air is moist, and 
the spores settle on the ground, plants, or other objects, 
instead of floating in the air. On the other hand, in 
winter, since very cold weather is gerierally dry, the 
number of air-germs increases. 
In summer, storms only purify the air for a very 
short time; within fifteen or eighteen hours alter the 
rain, the germs reappear, and are five to ten times 
more numerous than befure. It seems that storms 
give an energetic impulse to the production of moulds. 
If we turn to consider microbes, strictly so called, 
or the bacteria which are the causes of malicnant 
diseases, research becomes more difficult, on account 
of their smaller size and great transparency. An 
expedient is necessary to reveal their presence and 
enable us to count them accurately: this expedient 
consists in staining them by various processes, of 
which we shall speak when we come to discuss the 
micrographic study of drinking-water. Miquel prefers 
the process of filtration of the air invented by 
Pasteur, which consists in passing the air and aqueous 
vapour into such sterilized liquids as are favourable 
to the nutrition of microbes. 
