THE CHEMISTRY OF CLEANING. in 



and also the carbonates of iron and magnesium. If we examine a 

 spring water, we shall find that its dissolved impurities can be 

 divided into two classes : for instance, taking the Kent water sup- 

 plied at Greenwich, and obtained from deep welk in the chalk, 

 we find its saline constituents in grains per gallon are : 



Calcic carbonate ] 6-30 



Calcic sulphate 5"37 



Magnesic sulphate 0'93 



Magnesia nitrate 1-20 



Sodic chloride 2-64 



Sodic nitrate 1'21 



Silica, alumina, etc 097 



And of these the calcic sulphate, magnesium, and sodium salts 

 are dissolved by the solvent action of the water in the same way 

 that sugar would be, while the chief impurity, calcic carbonate, 

 is scarcely at all soluble in the water itself, 16,000 parts of pure 

 water only dissolving one part of the carbonate, but is readily 

 soluble in the carbonic acid, in the water which converts it into 

 soluble calcic bicarbonate. 



In the household, waters are roughly classified as hard or soft 

 waters, and the property of hardness manifests itself, as a rule, to 

 the householder by its action upon soap, and also by the amount 

 of " fur " which it causes in the kettle, these actions being due to 

 calcic bicarbonate, calcic sulphate, and the magnesium salts pres- 

 ent in it, all of which act upon soajD and cause it to curd instead 

 of forming a lather by converting the soluble sodic oleate and 

 stearate into insoluble lime salts, while the bicarbonate by de- 

 composing and depositing " chalk " causes the fur. 



A more careful examination, however, reveals the fact that 

 this property of hardness owes its origin to two different causes ; 

 for if we boil water until all the bicarbonate is broken up and the 

 calcic carbonate deposited, the clear water left behind it is yet 

 hard, though to a far less extent, and will still decompose a cer- 

 tain proportion of soap. The hardness which can be got rid of by 

 boiling is due to bicarbonate of lime, and sometimes also bicar- 

 bonate of magnesia, and is called " temporary hardness," while 

 the hardness left after boiling the water is due to calcic sulphate 

 and the soluble magnesium sulphate, chloride and nitrate, and is 

 called " permanent hardness." 



The relative hardness of waters is estimated by the amount of 

 soap they will destroy i. e., convert from the form of soluble sodic 

 oleate and stearate into the condition of insoluble oleates and 

 stearates of lime ; and one grain of calcic carbonate, or its equiva- 

 lent in sulphate or salts of magnesia, dissolved in a gallon of water, 

 is said to equal 1 of hardness. 



