THE MICKOBES OP 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 generally dry, the 

 number of air-germs increases. 



In summer, storms only purify the air for a very 

 short time ; within fifteen or eighteen hours after the 

 rain, the germs reappear, and are five to ten times 

 more numerous than before. 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 malignant 

 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. 



H 



