BACTERIA IN WATER 137 



examination of water may be undertaken, though these may 

 be of a purely scientific character, they usually aim at contribut- 

 ing to the settlement of questions relating to the potability of 

 waters, to their use in commerce, and to the efficiency of pro- 

 cesses undertaken for the purification of waters which have 

 undergone pollution. The last of these objects is often closely 

 associated with the first two, as the question so often arises 

 whether a purification process is so efficient as to make the 

 water again fit for use. 



Water derived from any natural source contains bacteria, 

 though, as in the case of some artesian wells and some springs, 

 the numbers may be very small, e.g. 4 to 100 per c.c. In rain, 

 snow, and ice there are often great numbers, those in the first two 

 being derived from the air. Great attention has been paid to 

 the bacterial content of wells and rivers. With regard to the 

 former, precautions are necessary in arriving at a judgment. 

 If the water in a well has been standing for some time, 

 multiplication of bacteria may give a high value. To meet this 

 difficulty, if practicable,- the well ought to be pumped dry and 

 then allowed to fill, in order to get at what is really the im- 

 portant point, namely, the bacterial content of the water entering 

 the well. Again, if the sediment of the well has been stirred 

 up a high value is obtained. Ordinary wells of medium depth 

 contain from 100 to 2000 per c.c. With regard to rivers very 

 varied results are obtained. Moorland streams are usually very 

 pure. In an ordinary river the numbers present vary at 

 different seasons of the year, whilst the prevailing temperature, 

 the presence or absence of decaying vegetation, or of washings 

 from land, and dilution with large quantities of pure spring 

 water, are other important features. Thus the Franklands 

 found the rivers Thames and Lea purest in summer, and this 

 they attributed to the fact that in this season there is most 

 spring water entering, and very little water as washings off land. 

 In the case of other rivers the bacteria have been found to be 

 fewest in winter. A great many circumstances must therefore 

 be taken into account in dealing with mere enumerations of 

 water bacteria, and such enumerations are only useful w 7 hen 

 they are taken simultaneously over a stretch of river, with 

 special reference to the sources of the water entering the river. 

 Thus it is usually found that immediately below a sewage 

 effluent the bacterial content rises, though in a comparatively 

 short distance the numbers may markedly decrease, and it may 

 be that the river as far as numbers are concerned may appear 

 to return to its previous bacterial content. The numbers of 



