WATER SOLVENT ACTION OF WATER, ETC. 55 



cium salts, though the iron precipitates as the hydroxide 

 when sodium carbonate is the precipitant. Iron is objec- 

 tionable in laundry work. Sodium carbonate is objection- 

 able in water used in locomotive boilers, because it induces 

 foaming; it is also objectionable in water for irrigation 

 purposes, causing the accumulation of alkali in the soil. 

 Permanently hard water affects the paper maker, the 

 tanner, the bleacher, and the dyer. 



45. Filtered Water. Thorough filtering makes water 



FIG. 17. An effective sand filter. (Drawing by Mr. T. C. Hough.) 



more attractive for household use, as color, odor, turbidity, 

 sediment, and to some extent, hardness may be removed 

 and an infected water may be made safer for drinking 

 purposes. City water-supplies are often treated in this 

 way and improved. Fig. 17 shows an effective sand filter. 

 The bottom of the filter, A, consists of puddled clay 2 feet 

 in thickness, built in with stones 8 inches in thickness; 

 layer B consists of coarse, angular stones and is about 30 

 inches in thickness. The next layer, C, consists of 6 inches 

 of smaller stones and over this is placed D, composed of 



