Germs in the Air of Different Places 49 



the nutrient gelatin and, developing there, produce 

 colonies whose number can be counted, and the char- 

 acter of whose organisms can be further investigated. 

 In the second method, the liquid is mixed with nutrient 

 gelatin, the latter spread out on sterile plates of gelatin, 

 and the colonies that gradually appear are counted and 

 studied in a similar manner. Investigations of this 

 nature, with the overwhelming evidence they bring 

 as to the occurrence and the distribution of bacteria, 

 and of other microorganisms in the atmosphere, soon 

 swept away the last vestige of belief in the theory of 

 spontaneous generation. 



INFLUENCE OF LOCALITY 



It was observed by Pasteur that the proportion of 

 flasks that showed, growth was variable and strongly 

 influenced by the locality whence the air was taken. 

 The air from the city contained more germs than the 

 air from the country, while the air from mountains 

 was poor in bacteria. 



Bacteria in city air. — The more exact and extensive 

 researches of Miquel have confirmed the observations 

 made by Pasteur. Miquel found in several examinations 

 an average of 3,480 bacteria per cubic meter in the air 

 of the Paris stteets, an average of 7,420 in the laboratory 

 air, an average of 36,000 in the air of old houses, and an 

 average of 79,000 in the air of one of the Paris hospitals. 

 These figures are important in showing how . human 

 activities lead to an increase of the bacteria in the air. 

 The number of germs in human habitations is undoubt- 



