CHAPTER VI. 

 DISTRIBUTION OF BACTERIA. 



§1. BACTERIA IN THE AIR 



As there is nothing in the air which can serve as food for bacteria, 

 the number of these organisms in the atmosphere of any particular 

 place depends altogether on the underlying surface. If the constitution 

 of the latter is of such a nature that it cannot support bacterial life, or 

 if the bacteria that are in it cannot escape into the atmosphere, then 

 we find the atmosphere altogether devoid of bacteria. Thus it has 

 been found that at the top of Mont Blanc there are only 4-11 bacteria 

 in 1000 litres of the atmosphere, that there are any at all being due 

 to the fact that a few are periodically blown up from the valleys. In 

 places like the Arctic and Antarctic regions, and in the middle of great 

 oceans and of great deserts, the atmosphere contains no bacteria. On 

 the other hand the greatest number of bacteria will be found in the 

 atmosphere of those places where conditions favour bacterial activity 

 in the underlying surface, and also where the conditions are such 

 that it is possible for the bacteria to escape into the atmosphere. 

 The air of sewers is singularly free from bacteria, even although 

 sewage contains many millions of bacteria per cubic inch, because 

 of the moistness of the surfaces, within which the sewage is en- 

 closed. The greatest number' of bacteria is found in the atmosphere 

 of large towns, and especially in those places where a large number 

 ■of people are congregated in a small space and under insanitary 

 conditions. 



On the whole, the average number of bacteria in the atmosphere 

 is very small, and 100 per cubic metre has been accepted as a fair 

 average. It is thus estimated that a person who has lived up to 

 the age of 70, has inhaled about 25,000,000 bacteria. A number far 



