688 DISTRIBUTION AND HABITAT OF THE BACTEBIA. 



multiplication during their transport through the air. 

 Further, bacteria as a rule only pass into the air from 

 bacterial colonies which are thoroughly dry and are 

 broken into fragments by external force. Nageli has 

 shown that even strong currents of air are unable to 

 detach bacteria from moist surfaces ; it is only when 

 there is at the same time a spurting up of the fluid by 

 waves, or by violent agitation (mill wheels, washing, &c.), 

 or by the formation of bubbles, that particles of water, 

 and with them bacteria, can be carried by currents of air 

 Only over short distances. Even when a colony of bacteria 



drybacteria dries up it cannot at once become loose, so that portions 

 pass into the C0 uld pass into the air ; on the contrary, the dried bac- 

 teria generally adhere very firmly to the substance beneath, 

 and it is only by breakage due to external violence, or to 

 the eifect of temperature, that small light particles are 

 loosened and can be carried away by currents of air. 



Former observers had, it is true, at times obtained evidence 

 of detachment of bacteria by feeble currents of air, and even 

 from moist surfaces, but those experiments were not made 

 with pure cultivations of bacteria, nor by the help of solid 

 nutrient substrata, and hence bacteria entering through in- 

 sufficient joints might easily cause deception and lead to the 

 idea that they had become detached from the surfaces of the 

 fluids investigated. At the present time the experiments 

 can be readily repeated with completely uniform results if 

 we work with pure cultivations of the rarer forms of bacteria 

 and are thus able to distinguish organisms which have come 

 accidentally from without, from those which were present on 

 the surface of the nutrient substrata. 



C 



Difference in The bacteria, once they have passed into the air, float 

 or are carried about by currents of air for a varying time. 

 In addition to differences in the strength of the currents, 

 the size and weight of the floating particles is of especial 

 influence in this respect. Gross particles of dust, such 

 as one sees with the naked eye by any mode of illumina- 

 tion, soon fall in quiescent air along with the bacteria 

 adhering to them ; the smaller or so-called solar specks 

 float more easily, and are carried about by slight 

 currents. Finally, the minute collections of bacteria or 

 individual bacteria which are never visible to the naked 



