32 BACTERIOLOGICAL EXAMINATION OF WATER. 



already given show how impossible it is to fix any standard for 

 shallow wells. Unless properly covered, imperviously steined 

 to a sufficient depth and surrounded by at least an acre of 

 virgin ground, shallow wells must be considered dangerous 

 sources of supply. The number of micro-organisms in water 

 from such a source is also liable to extreme variation from 

 climatic conditions independently of pollution, so that practically 

 local standards are of little value. 



(d) The supply is derived from a river. The bacterial con- 

 tents of rivers vary within very wide limits, depending upon 

 seasons of the year and proximity to towns, &c. As a rule, 

 they are dangerous sources of supply, for it appears that there 

 are few, if any, rivers which by natural processes can purify 

 themselves sufficiently to justify their consumption without 

 artificial treatment. 



