Although springs form some of the main feeders of rivers, there is almost invariably found in the latter a less weight of solids in the same volume, owing to dilution with rain-water, etc., and to the diffusion into the atmosphere of the free carbonic anhydride, and consequently the precipitation of such bodies as the carbonates of lime and magnesium which the water is only capable of retaining in solution in presence of that gas. The quality of the gaseous bodies present is very similar to those contained in spring-water; but the dissolved organic matter is generally very much greater, owing to contact with decaying leaves, plants, etc., and to the influx of land drainage.

The nature of this organic matter is a subject of the greatest moment, as upon it principally depends its suitability for drinking purposes. Animal organic matter is far more objectionable than vegetable organic matter, as its products of decomposition not unfrequently give rise to typhoid fever and other epidemic diseases. The oxygen dissolved in the water serves to a great extent as a purifying agent, acting upon the putrescent matter, and forming as final compounds carbonic anhydride, water, ammonia, nitrites and nitrates. Hence it follows that, as the free oxygen in the water is being constantly utilized in destroying the organic matter, the ratio of the dissolved oxygen to the nitrogen will vary; and. so by ascertaining these ratios we have an indication of the water's purity. The amount of mechanically-suspended particles in river-waters is also usually much greater than in spring-waters.