QUALITATIVE BACTERIOLOGICAL ANALYSIS. 161 



grow more quickly under anaerobic than aerobic conditions. 

 The exact conditions under which these organisms find, in 

 nature, temperatures favourable to their growth, are not yet 

 known ; they may possibly be of a chemical nature. Houston 

 found that thermophylic bacteria were present in London crude 

 sewage in abundance. These organisms are, however, found in 

 soils which have not been polluted recently, so their presence in 

 potable waters cannot be said to point to recent contamination. 

 But as all the workers on this subject have failed to detect 

 thermophylic bacteria in water from wells and springs, the 

 presence of these organisms in supplies derived from such sources 

 may safely be assumed to indicate gross contamination. Their 

 absence, however, cannot testify to purity or safety. 



