WATER SOLVENT ACTION OF WATER, ETC. 57 



boiler tubes. Bicarbonates and sulphates of calcium and 

 of magnesium dissolved in water make up, generally, at 

 least 90 per cent of its hardness. The presence of the 

 two bicarbonates constitutes what is known as temporary 

 hardness, which is more easily handled than permanent 

 hardness; the presence of calcium sulphate and of mag- 

 nesium sulphate constitutes permanent hardness. 



FIG. 19. Scale removed from a boiler. (From Power, Dec. 22, 1914.) 



The purification of boiler water may be accomplished 

 by the methods given for household water, page 54. 

 Temporary hardness may be removed by heating the 

 supply in a tank before it enters the boiler, by means of 

 waste steam or by any desirable method. This heating 

 converts the rather soluble bicarbonate of calcium or of. 

 magnesium into the much less soluble normal carbonate, 

 by expelling carbon dioxide, according to the following 

 formula : 



