VIII.] COMPOSITION OF NATURAL WATERS. 127 



of polluted water to the action of the atmosphere, which 

 is accomplished in a running stream, the organic matter 

 is oxidised, and may thus be eventually converted into 

 products which are perfectly harmless ; in other words, 

 a river is competent to effect its own purification unless 

 overtaxed with pollution. 



Although the organic impurities, or those derived from 

 animal and vegetable sources, thus suffer considerable altera- 

 tion as the river flows along, the saline matters, on the other 

 hand, remain unaffected, save in so far as they may be 

 consumed in supplying mineral matter to the organisms 

 which inhabit the river. The mineral matter is therefore, 

 for the most part, borne onwards by the river and finally 

 discharged into the sea. The sea consequently becomes 

 the ultimate receptacle for all the saline matter washed out 

 of the land and brought down by rivers. And yet the water 

 of the sea differs considerably in chemical composition from 

 that of rivers or of springs. Whilst a gallon of Thames 

 water contains in solution about 21 grains of saline matter, 

 a gallon of sea-water will contain something like 2,400 

 grains. In fact, the proportion of solid matter in sea- water 

 reaches as high as 3 1 to 4 per cent. It is needless to 

 remark that most of this saline matter consists of common 

 salt, such as we use at table, — a salt known to chemists as 

 chloride of sodium, since it consists of two elements, namely, 

 the gas ch/orifie and the metal sodium. Out of the 2,400 

 grains of mineral matter in a gallon of sea water, nearly 

 2,000 grains will consist of this common salt. 



As an example of the composition of sea-water, the 

 following analysis 1 of water from the British Channel may 



1 An analysis, by Schweitzer, in the I-hilosophka! Magazine, vol. xv. 

 p. 58. Recalculated to bring it into comparison with the analysis of 

 Thames water on p. 125. 



