178 



SCIENTIFIC NEAA^S. 



[Oct. 1st, i£ 



SOFTENING WATER. 



WE are all familiar with the disagreeable effect of wash- 

 ing in " hard " water, and we know that such water 

 is bad for making tea with. We know also that rain-water 

 is soft, and that the hardness of water is due to the presence 



rary hardness relates to the carbonates which are held in 

 solution by the carbonic acid in the water, and can be re- 

 moved either by the addition of milk of lime, with which 

 the carbonic acid then combines, or by boiling the water, 

 when the carbonic acid is driven off and the carbonates 

 which were in solution are precipitated. The permanent 



of salts in solution which have been taken up during 

 the passage of the water through various strata of the 

 earth. 



The salts which produce this hardness are usually the 

 carbonates and sulphates of calcium and magnesium. 

 Chemists, however, divide the hardness into temporary and 

 ■permanent, and in the purification of water it is -most im- 

 poitant that both should be taken into account. The tempo-' 



hardness is caused by the sulphates which remain after 

 the carbonates have been removed, and these require 

 special treatment. The total hardness of water is ex- 

 pressed in degrees, based on the number of parts of calcium 

 carbonate, or of the corresponding magnesium or other 

 calcium salts, which are contained in 70,000 parts of the 

 water. The Thames water, for instance, has a total hardness 

 of 15°, or contains in solution 15 grains of carbonate of lime 



