FILTRATION RAIN AND RIVER WATER. 



substances which it holds in solution. The fresh water of a lake 

 has a colour different from that of the salt water of the sea. How 

 far the colour of water may arise from the various substances 

 which it holds in solution is difficult to decide, inasmuch as we 

 caiinot obtain a sufficient quantity of water absolutely pure to bo 

 enabled to ascertain its proper colour. 



25. It appears from what has been explained that filtration 

 only disengages from water the solid impurities which may be 

 mechanically mixed with or suspended in it ; and if all water in 

 the natural state holds in solution more or less foreign matter, it 

 may be asked how water absolutely pure can be obtained ? 



It must be observed that, for all ordinary purposes, water 

 chemically pure would be less suitable than such water as is com- 

 monly obtained. For alimentary purposes, absolutely pure water 

 would be neither agreeable nor sanitary. For culinary and 

 domestic purposes such purity is not needed. 



26. Of all water found in the natural state, rain water is the 

 purest. But this, as commonly obtained, having first fallen on 

 the roofs of buildings, and then passed through pipes and conduits 

 to the reservoirs in which it is collected, takes up and dissolves 

 more or less of the impurities formed upon the surfaces over which 

 it passes. To obtain rain water in perfect purity, it must therefore 

 be received directly as it falls in clean vessels. But even then it is 

 found to be impregnated more or less with air, and especially with 

 carbonic acid, which it absorbs from the atmosphere, Minute por- 

 tions of ammoniacal salts are also found in it, and if it fall near the 

 sea, it has generally a small portion of common salt in solution. 

 Rain which falls during thunder storms has often traces of nitric 

 acid, formed probably by the effect of the atmospheric electricity. 



27. Xext to rain water, river water is the purest. The 

 Thames water, where it is not polluted by the drainage of the 

 metropolis, is found to contain no more than two grains of foreign 

 matter in solution in a pint. The matter which it thus holds in 

 solution is principally carbonate and sulphate of lime, common salt, 

 chloride of magnesium, and animal matter. A gallon of Thames 

 water in its most impure state, when properly filtered, does not 

 contain more than twenty-four grains of earthy or saline matter, 

 and in its purest state not less than sixteen grains. 



When water is contaminated by animal and vegetable matter, 

 if kept for some time, it undergoes a spontaneous purification, 

 losing its offensive odour and colour, and depositing more or less 

 sediment. Water for the supply of ships is well known to undergo 

 this process of purification by fermentation, and the larger the 

 quantity of destructible matter suspended in it, the more complete 

 and rapid is its purification. A preference is given to Thames 



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