388 SMITH'S INTERMEDIATE CHEMISTRY 



Casts are made smooth and non-porous (" ivory " surface) 

 by a coating of paraffin which fills the pores. Excellent imita- 

 tions of bronze or other castings are produced by rubbing with 

 pulverized metals. 



What Makes Water Hard. All natural waters except rain 

 water, which is " soft," contain salts of calcium and magnesium 

 in solution and are more or less " hard." These salts are dissolved 

 by the water in its passage over and through the soil. 



Although limestone is very insoluble in pure water (0.013 g. 

 per liter), yet it interacts with the carbonic acid contained in all 

 natural waters, giving calcium bicarbonate which is about thirty 

 times more soluble under atmospheric conditions: 



C0 2 + H 2 <=> H 2 C0 3 + CaC0 3 <= Ca(HC0 3 ) 2 . 



When the water is boiled, these actions are all reversed. The 

 carbon dioxide is driven out of solution, the carbonic acid is 

 decomposed, and the calcium bicarbonate gives calcium carbon- 

 ate, most of which is at once precipitated. Iron carbonate is 

 also held in solution as bicarbonate Fe(HCO 3 ) 2 and is precipi- 

 tated as FeCOs by boiling. These two bicarbonates constitute 

 temporary hardness. Their decomposition causes the " fur " in 

 a kettle. 



The sulphates of calcium (solubility 2 g. per liter) and of mag- 

 nesium (very soluble) are also commonly found in natural waters. 

 These salts are not altered by boiling and, along with magnesium 

 carbonate (sol'ty 1 g. per 1.) and calcium carbonate (sol'ty 0.013 g. 

 per 1.), give permanent hardness to the water. 



Consequences of Hardness in Water. When hard water 

 is used in a steam boiler, the salts, of course, are not carried off 

 with the steam, but accumulate amazingly as fresh water is in- 

 jected and steam alone is drawn off. In time, heavy deposits 

 of boiler crust settle on the tubes of the boiler, and interfere with 

 the transference of heat from the metal to the water. One-fourth 



