160 BACTERIA IN" WATER 



sewage, the presence of these in a water may be sought for. The methods 

 are those which have already been given (p. 158). 



Much work has been devoted to the question of these faecal streptococci 

 presenting specific characters by which they could be differentiated 

 from other streptococci. Houston has found that the prevailing type 

 of organism here is one which produces acid and clot in milk, reduces 

 neutral-red, and ferments saccharose, lactose, and salicin. It corresponds 

 to the streptococcus fcecalis of Andrewes and Horder. The important 

 point in this connection is to recognise that streptococci of such a type 

 exist in great numbers in human fseces, and that when in any circum- 

 stances fsecal contamination is suspected, the isolation of streptococci 

 strengthens the suspicion. 



With regard to the objects with which the bacteriological 

 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 

 processes 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 numerical count. To 

 meet this difficulty the well ought, if practicable, to be pumped 

 dry and then allowed to fill, in order to get at what is really the 

 important 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 often fairly pure. In an ordinary river the numbers 

 present vary at different seasons of the year, whilst the pre- 

 vailing 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 



