DISTRIBUTION AND HABITAT OF THE BACTERIA. 689 



eve, and of which the weight represents the billionth of 

 u gramme or less, do not become deposited to any great 

 extent even in quiescent air. We must also suppose 

 that these minute bodies are surrounded by a condensed 

 coating of air consisting chiefly of the vapour of water, 

 and forming as it were a parachute and mantle which 

 renders floating easier (Niigeli). 



From these observations and considerations we at Local differ- 

 once arrive at some laws as to the local and seasonal nnmbe? of"air 

 distribution of bacteria in the air. Everywhere where germs. 

 there is a plentiful development of bacteria on the sur- 

 face of the soil, and where complete dessication of super- 

 ficial colonies takes place, there will be a considerable 

 number of bacteria in the air. Where there is no oppor- 

 tunity for the growth of bacteria (in deserts, on high 

 mountains, &e.), or where the surface is always moist 

 (over the sea), the air will be almost or entirely free from 

 bacteria. Nothing certain is known as to the distances to 

 which dry but living bacteria can be carried by the wind ; 

 from the extraordinary distances over which other par- 

 ticles of dust can be carried, we may conclude that when 

 opportunity offers the bacteria may be carried over very 

 considerable areas. This circumstance naturally tends to 

 destroy to a considerable extent local differences in the 

 amount of bacteria in the air ; nevertheless, by far the 

 greater number of the air germs will always originate 

 from local sources. 



It cannot be decided, and it is also of subordinate impor- 

 tance, whether the few bacteria which have been found on 

 high mountains have their origin in winds or in the body, 

 the clothes and utensils of the observers, or in insignificant 

 local sources ; and of equally slight importance is the hunt 

 for bacteria in the air of the highest mountains and on the 

 most distant seas, which has been carried on for the last few 

 vears, has been entirely wanting in proper aim and has more 

 reference to the sensational public than to scientific interest. 



Variations at different times in the number of air Variations at 

 germs are, apart from the varying quantity of available 

 bacterial colonies, dependent in the first place upon the germs, 

 conditions which favour the passage of new bacteria 



44 



