Drinking- Water and some of its Impurities. 115 



the surface of the earth without taking with it much solu- 

 ble mineral matter. Rivers are more likely to be charged 

 with suspended impurities for the reason that their waters, 

 which have not been filtered through the soil, carry with 

 them a certain quantity of mud and organic matter. 

 When water flows into lakes and the sediment subsides, it 

 becomes clear, but in streams where the water runs rapidly, 

 it has no opportunity to deposit its sediment and it often 

 appears very turbid. 



The question arises, which of these impurities may be 

 allowed, and in what quantities. We will first consider 

 the lime and magnesian salts. In order to determine them 

 roughly the method devised by Dr. Clark and called 

 " Clark's Method" is generally employed. This method 

 is based upon the fact that when pure water is mixed with 

 a very small quantity of alcoholic solution of soap, and 

 shaken for a few minutes, a froth is produced which is 

 persistent for some miuutes ; while on the contrary, when 

 the water contains lime or magnesian salts in solution, 

 no such froth is produced, until a sufficient quantity of the 

 soap solution has been added to decompose them. The 

 amount of soap solution requisite to produce the froth 

 in water containing such salts is moreover to such au ex- 

 tent proportional to the amount of these salts in the water, 

 that with a soap solution of known volumetric value, the 

 degree of hardness is indicated by the quantity requisite to 

 produce the froth in a given quantity of water. The 

 hardness is expressed in degrees each of which represents 

 .01 grm. per litre, or one part per 100,000 of carbonate of 

 lime or its equivalent of some other lime or magnesian 

 salt. Water of less than 8°. 5 hardness is called soft water, 

 of more than 8°. 5 hard water. Water which is too hard 

 is less wholesome than that which may be called soft. 

 It is not so good for household purposes nor for boilers. 

 It also entails an enormous waste of soap. 



Natural waters contain carbonic acid dissolved in them, 



