414 EXAMINATION OF DRINKING-WATER. 



surface of the earth in certain localities in the form of springs. (3) The running 

 water of streams, rivers and brooks is generally much poorer in mineral matter 

 than that of wells or springs. 



Flowing on the surface spring- water soon gives off much of its carbon dioxid. 

 As the solution of many minerals is possible only in the presence of carbon dioxid, 

 insoluble precipitates of these substances must result. The water of wells and 

 springs is poor in oxygen, and on the other hand rich in carbon dioxid. The latter 

 gives it its refreshing and stimulating properties. For the same reason a generous 

 vegetable life is possible about springs, while on the other hand the existence in 

 spring-water and well-water of animal organisms requiring oxygen is extremely 

 limited. Freely running water, however, absorbs oxygen from the air, while giving 

 off carbon dioxid, and thus supplies the necessary conditions for existence to fishes 

 and other aquatic animals. River- water contains about from ^ to oV f its 

 volume of absorbed gases, which may be driven off by boiling or freezing. 



The water from wells and springs chiefly is used for drinking-purposes. River- 

 water (with which, unfortunately, some large cities must yet content themselves) 

 demands first a careful removal of the clay and other accidental impurities sus- 

 pended in it. It may be cleared and purified by means of large filter-beds made 

 of thick layers of sand mixed with charcoal. On a small scale the commercial 

 charcoal-filter can be used with advantage to clarify the water, the charcoal being 

 in addition disinfectant. In this connection it is a noteworthy fact that alum in 

 a dilution of o.oooi per cent, is able to clarify turbid water. 



EXAMINATION OF DRINKING-WATER. 



Drinking-water (even when viewed in thick layers) should be perfectly color- 

 less and clear, also without odor, which is best perceived by heating to 50 C., 

 with or without addition of sodium hydroxid. Moreover, it should not be too 

 hard, that is, not unduly rich in salts of calcium and magnesium. 



By the term degree of hardness is designated the content of compounds of cal- 

 cium and magnesium in 100,000 parts of water. A water of 20 degrees of hardness 

 contains, therefore, in 100,000 parts, 20 parts of calcium (calcium oxid) in com- 

 bination with carbon dioxid, sulphuric and hydrochloric acids (the small amounts of 

 magnesium need not be taken into consideration) . A good drinking-water should 

 not greatly exceed 20 degrees of hardness. To determine the degree of hardness 

 a titrated soap-solution may be used. This is shaken with the water to be exam- 

 ined, and the later that foam appears the harder is the water. The degree of 

 hardness exhibited by unheated water is designated its total hardness; that of 

 heated water its permanent hardness. By means of boiling, the calcium carbonate 

 principally is precipitated, as a result of the escape of the carbon dioxid. It is 

 on this account that boiled water becomes softer. 



Turbidity of the water following the addition of hydrochloric acid and barium- 

 chlorid solution indicates the presence of sulphuric acid, usually in combination 

 with calcium. 



As chlorin (always in combination with a metal) appears only in small quanti- 

 ties in pure spring-water, and as its presence in large amounts (apart from saline 

 springs, the vicinity of the sea, or factory-sewers) generally indicates a commu- 

 nication with water-closets or manure-heaps, the estimation of this is of 

 especial interest. For purposes of demonstration, 20 cu. cm. of water are mixed 

 with a few drops of nitric acid, and silver nitrate is added; a precipitate of silver 

 chlorid results. For quantitative estimation by titration there are necessary a 

 solution A of 17 grams of crystallized silver nitrate in i liter of water (i cu. cm. 

 of this solution precipitates 3.55 mgm. of chlorin as silver chlorid) ; and also a cold 

 saturated solution B of neutral potassium chromate. In testing, 50 cu. cm. of the 

 water to be examined are placed in a beaker, 2 or 3 drops of the solution B are 

 added, and then from a buret solution A is permitted to flow drop by drop until 

 the precipitate, at first white, remains red, even after stirring. If the number 

 of cubic centimeters of A used be multiplied by 7.1, the amount of chlorin con- 

 tained in 100,000 parts of water will be obtained. Example: if 50 cu. cm. required 

 2.9 cu. cm. of silver-solution, then 100,000 parts of water contain 2.9 X 7-* = 

 20.59 parts of chlorin. In good drinking-water the chlorin should not exceed 

 15 mgm. in i liter. 



If 50 cu. cm. of water are acidulated with a little hydrochloric acid, then 

 ammonia added in excess, and to this a solution of ammonium oxalate, the white 

 precipitate obtained is calcium oxalate. According as the resulting turbidity is 

 only slightly cloudy or markedly milky it is known whether the water is soft 



