QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 31 



local standards for comparison will be obtained which will have 

 great practical value. For general reference water-supplies 

 may be arranged in the following groups : (a) spring and deep 

 well waters ; (b) upland surface waters ; (c) shallow well waters ; 

 (d) river waters. 



(a) The supply is obtained from main springs and deep wells. 

 These two sources are classed together because they both repre- 

 sent water which has percolated through a considerable depth 

 of ground ; they are really filtered supplies containing, as a rule, 

 very few bacteria. Tiemann and Gartner consider that water 

 derived from these sources should never contain more than 50 

 micro-organisms per c.c., and the analyses previously given 

 show that this is a reasonable limit ; as the water has percolated 

 through a considerable depth of ground it keeps a fairly 

 uniform temperature throughout the year, and is very little 

 influenced by climatic conditions. Consequently if deep wells 

 and springs are kept free from surface pollution, and the out- 

 crops of the water-bearing strata are above suspicion, there is 

 every reason to expect a constant bacterial composition of these 

 supplies. 



(b) The supply is derived from an upland surface. The 

 water is usually collected in an impounding reservoir and may 

 be distributed without filtration. Such a supply must be 

 studied locally by determining the bacterial contents of un- 

 polluted feeders at different times of the year. If to this study 

 there be added an examination of the reservoir at its upper end 

 where the various feeders enter, and at the lower end where the 

 water is discharged after sedimentation has helped to reduce the 

 number of micro-organisms, a very good idea of the true bac- 

 terial contents of the supply will be obtained. Tables can then 

 be made for reference which will form an effective control of 

 the supply. It often happens, however, that the upland surface 

 grounds are obviously open to pollution from human habita- 

 tions, &c. In such a condition of things it will be of little use 

 to determine the average bacterial contents of the feeders, 

 because even daily examinations would not enable the supply to 

 be effectively safe-guarded. The water must then be subjected 

 to the artificial conditions of filtration. 



(c) The water is derived from shallow wells. The analyses 



