152 BACTERIA IN AIR 



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 

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

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

 streptococcus salivarius (q.v.) of Andrewes and Horder 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. equinus, 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 

 germs was present. The value of this as a practical method has yet to be 

 determined. 



SOIL. 



The investigation of the bacteria which may be found in the 

 soil is undertaken from various points of view. Information 

 may be desired as to the change its composition undergoes by 

 a bacterial action, the result of which may be an increase 

 in fertility and thus in economic value. Under this head may 

 be grouped inquiries relating to the bacteria which convert 

 ammonia and its salts into nitrates and nitrites, and to the 

 organisms concerned in the fixation of the free nitrogen of the 

 air. The discussion of the questions involved in such inquiries 

 is outside the scope of the present chapter, which is more con- 

 cerned with the relation of the bacteriology of the soil to questions 

 of public health. So far as this narrower view is concerned, soil 

 bacteria are chiefly of importance in so far as they can be washed 

 gut of the soils into potable water supplies. An important aspect 



