70 Agricultural Chemistry. 



3. Sodium or potassium chloride. 



4. Alkaline silicates. 



5. Dissolved gases as oxygen, nitrogen and especially carbon 

 dioxide. 



Calcium and magnesium carbonates are almost insoluble in 

 water, but if the water contains carbon dioxide, the readily sol- 

 uble bi-carbonates of calcium and magnesium are formed. 



Such action occurs in all lime-stone districts and the removal 

 of the rock by solution gives rise to the caves and underground 

 water courses so common in such localities. The great Mammoth 

 Cave of Kentucky and Perry Cave of Northern Ohio are illus- 

 trations of such action. 



When such water is boiled the bi-carbonates are decomposed, 

 losing part of their carbon dioxide, and normal carbonates are 

 again formed. These are insoluble and consequently appear as 

 a precipitate. In many cases the precipitated calcium or mag- 

 nesium carbonate forms a firmly adherent coating or crust upon 

 the bottom or sides of the kettle or boiler. 



Calcium and magnesium sulphates are soluble in water, the 

 former to the extent of about 1.7 grams per liter (1 oz. in 18 

 quarts of water). Waters containing calcium or magnesium 

 compounds are known as "hard" waters, and have a peculiar 

 and well known action on soap. The latter is essentially a sodium 

 salt of the fatty acids, as stearic, palmitic and oleic acids. These 

 acids are the constituents of our principal fats and it is the com- j 

 mon practice of every good housewife to save the fat "scraps' 7 

 for the home soap-making. The sodium and potassium salts of 

 the fatty acids are soluble in water, but the calcium and mag- 

 nesium salts are insoluble. For water to form a lather with 

 soap or properly exercise its cleansing power, it is necessary that 

 the water should contain in solution some of the sodium or potas- 

 sium salts of the fatty acids. When a small quantity of soap is 

 dissolved in hard water, the calcium or magnesium present in 

 the water displaces the sodium or potassium and gives a curdy, 

 flocculent precipitate of the calcium or magnesium salts of the 



