In judging the quality of water that is to be used for drinking purposes, it is necessary to take into account the source from which the water is procured, because the surroundings of the gathering ground have great influence upon the composition of the water obtained therefrom. Drinking water is obtained (1) from upland surfaces and collected in reservoirs; (2) from springs; (3) from deep wells; (4) from shallow wells; (5) from rivers and streams; and smaller quantities from other sources. Unpolluted waters from any of the above named sources will vary in composition according to the nature of the soil or rock on which the water is collected, or over which it flows, or through which it percolates; but the figures given in the following tabular statement may be taken as examples of the average composition of water from the five sources of supply referred to above. The figures are compiled from the sixth report of the Rivers Pollution Commission, 1871, and from other sources.

The organic carbon and nitrogen in upland surface water, and in river water of very excellent quality, would not be more than 250 and .025 part respectively. Waters highly polluted with sewage contain more than .3 of organic carbon, .03 of organic nitrogen, and .02 of free NH3; nitrogen as nitrates and nitrites maybe very low or very high according to the amount of aeration the water has received, from none to more than .5 ; chlorine 5 parts per 100,000. No amount of filtration would render fit for drinking purposes water that is polluted by sewage. In such cases the polluting matter is, to a large extent, held in solution, and cannot therefore be filtered out. A filter deals only with matter held in suspension, and possesses no other protective power. Water that is polluted by matters held in solution can be purified only by chemical action artificially induced and promoted, or by such natural chemical agencies as are supplied by air and sunlight and friendly bacteria.