62 



OUTLINES OF BACTERIOLOGY 



§ 6. GRAVITY. 



The earth exerts an attraction for bacteria as for all other sub- 

 stances. The effect of this influence is indicated by the number of 

 bacteria at different altitudes, the number nearer the ground being 

 greater than at a higher level. This can be demonstrated by the 

 following tables : 



organisms per litre. 



bacteria per 10 litres. 



It is therefore seen that the nearer we are to the ground the greater 

 is the number of microbes, and that at high altitudes there are 

 practically none at all, except such as are occasionally carried up by 

 air currents. At the top of Mont Blanc the examination of 100 litres 

 of air did not reveal the presence of a single microbe, and the total 

 number of organisms, including bacteria and moulds, was found to be 

 from four to eleven per 1000 litres. 



