690 DISTRIBUTION AND HABITAT OF THE BACTERIA. 



Influence of 

 dry winds. 



Effect of the 

 distribution 

 of the germs 

 over a great 

 extent of air. 



into the air ; and in the second place upon the factors 

 which influence the deposit of floating germs from the 

 air. Dry winds more especially lead to an increase in 

 the number of bacteria in the air. Even when the de- 

 ficiency of moisture in the air is not great, and when the 

 air is moist, drying of the most superficial layers may 

 occur at the exposed parts of the surface of the earth, 

 and dust may he carried into the air along with a certain 

 number of bacteria ; great drought lasting for some time 

 (as in the case of the east winds in our neighbourhood) 

 leads, however, to a much greater degree of drying ; every 

 corner of the streets, courts, and houses, the deeper layers 

 of the soil, &c., gradually become dry, and many more, 

 and also greater varieties of, bacteria, ultimately also 

 pathogenic bacteria, pass into the air in these places. 



In spite of this markedly favourable influence of dry 

 winds in increasing the number and kinds of air germs, 

 it is nevertheless possible that a cubic metre of the air in 

 our neighbourhood may scarcely show more germs than 

 in still, damp weather. For the dry winds will possibly 

 distribute the germs which are taken up over a much 

 greater area, and more especially carry them to a con- 

 Deposit of air siderable altitude. A greater amount of water in the 



S-ermsbythe , ,. . ,, , 



condensation atmosphere, on the contrary, more especially, however, 

 vapour * * ne occurrence of descending currents of moist air, and 

 in particular a great condensation of water vapour, must 

 lead to sinking of the particles of dust, and thus occasion, 

 in the first place, an increase in the number of germs in 

 the layers of air close to the surface of the earth, until 

 continued condensation and rains have eventually brought 

 back the greater number of the bacteria to the soil. In 

 the investigations made up to the present time, but little 

 attention has been paid to these conditions affecting the 

 distribution of the germs in the air, and especially to 

 the possibility of their deposition by condensation of 

 water vapour, although it is probable that a more inti- 

 mate knowledge of the mode of distribution of the air 

 germs will give us enlightenment as to the mode in which 

 many infections (e.g., malaria) occur. 

 Over estima- On the whole too great a role was formerly ascribed to 



t r on of the J 



