Ii6 PHYSIOGRAPHY. [chap. 



though present in very large proportions, may entirely 

 elude observation by the eye, the water remaining clear and 

 colourless. These soluble constituents, unlike the suspended 

 impurities, will not be deposited when the solution is allowed 

 to stand, nor will they be removed by the mere act of 

 filtration. All natural water — whether brook or river, lake 

 or sea — contains such dissolved matter, chiefly in the form 

 of the various compounds called salts ; but it varies con- 

 siderably, in character and in quality, in different varieties 

 of natural waters. 



It is not necessary to go far to seek the source of these 

 dissolved impurities. All the rocks of the earth, over 

 which the waters flow, or through which they drain, contain 

 mineral constituents more or less soluble in water. Water 

 is, in fact, an almost universal solvent, whether of solids, 

 liquids, or gases. River-water must therefore be regarded, 

 not as absolutely pure water, but rather as an extremely 

 weak solution of certain chemical compounds. What these 

 compounds are will now be explained. 



When natural water is evaporated, all its impurities, 

 except those which are volatile, are left behind, and the 

 vapour which rises is very nearly pure water. When the 

 vapour of water is condensed it reproduces pure water. But 

 such water rapidly absorbs both oxygen, nitrogen, carbonic 

 acid, and ammonia \ and, hence, the rain when it reaches 

 the earth is no longer pure water ; it has absorbed some of 

 the atmospheric gases. Rain-water, therefore, although the 

 purest of all forms of natural water, contains certain im- 

 purities which it has washed out of the atmosphere. The 

 oxygen of the air is more soluble than the nitrogen ; the 

 carbonic-acid gas is much more soluble than either oxygen 

 or nitrogen ; and the ammonia is far more soluble than 

 any of the other gases. Thus, under normal conditions of 



