DISTRIBUTION AND HABITAT OF THE BACTERIA. 687 



exhausted nutrient material. Where spore forms are 

 absent, preservation can possibly still occur if the exist- 

 ing conditions are such that the sensitive parasitic bac- 

 teria are not destroyed or overgrown by saprophytes. 

 Conditions of this kind are furnished, for example, where 

 the temperature is under +5 C.; and also (as will be 

 described more fully afterwards) in porous and moderately 

 moist soil. 



For the distribution of bacterial life on the surface of Transport of 

 the soil, it is also of importance that they should not be 

 restricted to the original place where they developed, but 

 should be transported over greater or less distances. 

 Currents of air and flowing water are the most important 

 means of transport; to a less extent, but in a great 

 variety of ways, they may also be carried by animals and 

 by the trade and intercommunication of mankind. 



When we examine our various surroundings as to the Occurrence 

 presence of bacteria, we find them, in the first place, in of bacteria^ 

 the air in very varying numbers. By the methods as the air - 

 yet emplo}'ed for these investigations, from 100 to 500 

 living bacteria have been found in every cubic metre of 

 the layers of the air which lie immediately above the 

 ground; in the air of dwelling-rooms they have been 

 found in very small numbers when all movement of the 

 air has been as much avoided as possible for some time, 

 while they are present in large numbers when the dust 

 has been raised by movements and other disturbances. 

 Direct microscopical examination of the collected air 

 germs, as well as the experiments on filtration of the air 

 (Hesse), have shown that the micro-organisms floating 

 in the air do not occur as isolated individuals, but that 

 numerous individuals of the same kind are as a rule united 

 together in chains and groups, or adhere to coarse par- 

 ticles and visible pieces of dust. 



The origin of the air germs must almost always be Origin of air 

 sought in the bacterial colonies on the surface of the fferms - 

 earth, for there is not sufficient moisture to permit 



