HARD AND SOFT WATERS 153 



produce diarrhoea and dysentery in animals drinking them. 

 Surface drainage and sewage are apt to contain vegetable 

 and animal parasites, bacteria, spores and ova, which give 

 rise to dangerous diseases in animals as well as in man. 



Even in potable waters the nature and proportion of the 

 solid constituents differ materially. Glasgow derives from 

 Loch Katrine the purest water supply of any large city 

 in the world, containing only three-fourths of a grain of 

 organic and one and a half grains of inorganic matters to 

 the gallon. The water of the Thames, supplied to part of 

 London, contains about three grains of organic and sixteen 

 grains of inorganic matters to the gallon. When the mineral 

 constituents, consisting of salts of calcium and magnesium, 

 exceed -5 <j\^)th part, the water is said to be hard, and is un- 

 suitable for many pharmaceutic and domestic purposes ; it 

 curdles or precipitates soap, instead of forming with it a 

 froth or lather ; it forms a brown encrustation on the kettles 

 or furnaces in which it is boiled ; it is not so well liked by 

 animals, and is apt to cause diarrhoea and other digestive 

 derangements, especially in subjects unaccustomed to it. 

 When the salts do not amount to ^-^th P ar ^ ^he water is 

 considered soft. 



The presence of the more dangerous organic and organised 

 impurities is discovered by several simple tests (1) Half a 

 pint of the water is well shaken in a clean, wide-mouthed 

 bottle ; when sewage is present an offensive smell will be 

 discovered on removal of the stopper or cork. (2) In a 

 tumbler of water two or three drops of sulphuric acid are 

 placed, and sufficient potassium permanganate to render 

 the water pink. When allowed to stand for fifteen minutes, 

 the water, if containing organic impurity, will have become 

 colourless. (3) Sewage contaminated water usually con- 

 tains common salt, which may be discovered by silver 

 nitrate producing milkiness (AgCl), which is not removed 

 by a few drops of nitric acid. 



For purifying water various methods are adopted. Sub- 

 sidence and decantation get rid of grosser mechanical par- 

 ticles. Filtration through sand, charcoal, gravel, or spongy 

 iron removes coarse and organic impurities, but filters may 

 become infected, and are not usually proof against bacteria. 



