470 



latter in medicine, in dyeing (delicate shades), and to replace the 

 cream of tartar (p. 431) in making baking powder. 



Sodium - alum, Na2S04,Al2(S0 4 )3,24H 2 0, ammonium - alum 

 (NH 4 ) 2 S0 4 , A1 2 (S0 4 ) 3 , 24H 2 O and chrome-alum K 2 S0 4 , Cr 2 (SO 4 ) 3 , 

 24H 2 are made in the same way, and crystallize in the same form. 

 The first two are used as sources of aluminium hydroxide, and the 

 last hi the " fixing bath " to harden the gelatine on photographic 

 films and plates. 



Water_ Purification Coagulation Process. The sus- 

 pended matter in water to be used for a domestic supply can be 

 coagulated into larger particles by introducing a small amount of 

 the gelatinous precipitate of aluminium hydroxide. These larger 

 particles, which adsorb also the greater part of the bacteria, settle 

 rapidly and the process therefore permits the use of relatively 

 small settling ponds. Aluminium sulphate, made from crude 

 bauxite, and lime are added to the water: 



3Ca(OH) 2 + Al 2 (S0 4 ) 3 -3CaS0 4 + 2A1(OH) 3 . 



If the water has much temporary hardness, lime is not required: 



3Ca(HC0 3 ) 2 + A1 2 (S0 4 ) 3 - 3CaSO 4 + 2A1(HC0 3 ) 3 . 



The carbonate of aluminium, being a salt of both a very weak 

 acid and a very weak base, if formed, would be instantly hydro- 

 ly zed : 



A1(HC0 3 ) 3 + 3H 2 -> A1(OH) 3 J, + 3H 2 CO 3 



so that aluminium hydroxide is precipitated. 



The few remaining bacteria are destroyed by the addition of 

 bleaching powder (p. 224). 



Crude ferrous sulphate JFeS0 4 (copperas) ... being in many places 

 cheaper than aluminium sulphate, is often used instead of the 

 latter. The lime precipitates ferrous hydroxide Fe(OH) 2 . This 

 is quickly oxidized to the red ferric hydroxide Fe(OH) 3 , which 

 coagulates the suspended matter. 



