DISTRIBUTION OF BACTERIA BY AIR 151 



nM|iihv ;ui air current moving at the rate of 1 centimetre per 

 second in l-rp them suspended, and the finer dust which can In- 

 kept in suspension by currents moving at from 1 to 4 milli- 

 metres per second. In the former case, when once the particles 

 settle they cannot be displaced by currents of air except when 

 these are moving at, at least, 5 metres per second, but the 

 brushing, shaking, or beating of objects may, of course, distribute 

 tin-in. In the case of the liner dust the particles will remain for 

 long suspended, and when they have settled can ' be more easily 

 displaced, as by the waving of an arm, breathing, etc. With re- 

 gard to infection by dust, a most important factor, how r ever, is 

 whether or not the infecting agent can preserve its vitality in 

 a dry condition. In the case of a sporing organism such as 

 anthrax, vitality is preserved for long periods of time, and great 

 resistance to drying is also possessed by the tubercle and 

 diphtheria bacilli ; but apart from such cases there is little 

 doubt that infection is usually necessarily associated with the 

 transport of moist particles, and is thus confined to a limited 

 area around a sick person. Among diseases which may occasion- 

 ally be thus spread, cholera and typhoid have been classed. 

 Considerable controversy has arisen with regard to certain out- 

 breaks of the latter disease, which have apparently been spread 

 by dusty winds, although we have the fact that the typhoid 

 bacillus does not survive being dried even for a short time. 

 It appears, however, that in such epidemics the transport of 

 infection by means of insects carried by the wind has not been 

 entirely excluded. 



As in the cases of the soil and of water, presently to be described, 

 attempts have been made to obtain indirect evidence of the contamination 

 ni the air from human sources. Thus Gordon has shown that certain 

 streptococci are common in the saliva ; these usually correspond to the 

 f'ococcus salirarius of Andrewes d Horder (q.v.) in that they grow 

 at 37 C., form acid and clot in litmus milk, reduce neutral-red, and fer- 

 ment saccharose, lactose, and raffinose. Andrewes and Horder also describe 

 another group, sir. cquinm, as common in London air, which they 

 think is there derived from horse dung. Thus the finding of streptococci 

 of the first group in plates exposed to air would indicate that a human 

 source was probable, and, if the observation were made on air from the 

 neighbourhood of a sick person, that risk of the dissemination of disease 

 gTins was present. The value of this as a practical method has yet to he 

 determined, 



Son.. 



The investigation of the bacteria which may be found in the 

 soil is undertaken from various points of view. Information 



