128 BACTERIA IN AIR, SOIL, AND WATER. 



to infection by dust, a most important factor, however, is whether 

 or not the infecting agent can preserve its vitality in a dry con- 

 dition. In the case of a sporing organism such as anthrax, 

 vitality is preserved for long periods of time, and great resist- 

 ance to drying is also possessed by the tubercle and diphtheria 

 bacilli ; but apart from such cases there is little doubt that infec- 

 tion is usually necessarily associated with the transport of most 

 particles, and is thus confined to a limited area around a sick 

 person. Among diseases which may occasionally be thus spread 

 cholera and typhoid have been classed. Considerable contro- 

 versy has arisen with regard to certain outbreaks 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 ; in 

 fact, the common house-fly comes strongly under suspicion of 

 being the carrier of infection during epidemics of typhoid fever 

 and cholera, where fecal discharges may have been carelessly 

 disposed of. 



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 am- 

 monia and its salts into nitrates and nitrites, and to the organ- 

 isms 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 ques- 

 tions of public health. So far as this narrower view is con- 

 cerned, soil bacteria are chiefly of importance in so far as they 

 can be washed out of the soils into potable water supplies. An 

 important aspect of this question thus is as to the significance 

 of certain bacteriological appearances in a water in relation to 

 the soil from which it has come or over which it has flowed. 

 In this country (Great Britain) these questions have been chiefly 



