PREFACE. 



IN this volume I have endeavoured to present in a simple form 

 the methods and results of a bacteriological examination of 

 water. The object of such an examination being to discover 

 whether a water is likely to be prejudicial to health, it is 

 obviously necessary to have clear ideas as to the hygienic value 

 of the various micro-organisms which have been isolated from 

 water. Great difficulties have been introduced into the subject 

 owing to the extreme variability of water bacteria. The micro- 

 scopical appearances of these micro-organisms are largely 

 influenced by the media on which they are growing, and the 

 cultural characteristics are greatly dependent on whether the 

 micro-organisms are enfeebled or not. It is also well known 

 that bacteria, which normally inhabit the human body, find in 

 water very unfavourable conditions for their existence ; con- 

 sequently, after a comparatively brief existence in this medium, 

 they often exhibit weakened forms when isolated on ordinary 

 bacteriological media. In order to simplify matters as much as 

 possible, the bacteria which occur in water have been arranged 

 in three classes, viz., (1) bacteria which are found in pure water ; 

 (2) bacteria which are common in sewage but rarely met with in 

 pure water ; and (3) bacteria which give rise to specific disease 

 in human beings. The micro-organisms in the first class are 

 very numerous, but many of them have been so imperfectly 

 described that it has been very difficult to group them in a 

 satisfactory manner. As the easiest solution of the difficulty, 

 and to facilitate reference, certain well-defined types have been 

 selected, and under each type have been described all those 

 bacteria which are probably varieties, the variations being caused 

 by differences in food, habitat, c. I have examined nearly all 

 the bacteria mentioned, and the descriptions are taken from 



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