SODIUM 



521 



furthest removed from the fire-grate ; it is then brought to the portion 

 nearest to the fire-grate, when it is stirred during heating. The 



FIG. 68. Eeverberatory furnace for th manufacture of sodium carbonate. P, grate. A, bridge. 

 M, hearth for the ultimate calcination of the mixture of sodium sulphate, coal, and calcium car- 

 bonate, wnich is charged from above into the part of the furnace furthest removed from the fire F. 

 P, P, doors for stirring and bringing the mass towards the grate F by means of stirrers R. At the 

 end of the operation the semifused mass is charged into trucks C. 



partially fused mass obtained at the end of the process is cooled, and 

 then subjected to methodical lixiviation I8 to extract the sodium car- 



15 Methodical lixiviation is the extraction, by means of water, of a soluble substance 

 from the mass containing it. It is carried on so as not to obtain weak aqueous solutions, 

 and in such a way that the residue shall not contain any of the soluble substance. This 

 problem is practically of great importance in many industries. It is required to extract 

 from the mass all that is soluble in water. This is easily effected if water be first poured 



FIG. 69, 



-Apparatus for the methodical lixiviation of black ash, &c. Water flows into the tanks 

 from the pipes r, r, and the saturated liquid is drawn off from c. o. 



on the mass, the strong solution thus obtained decanted, then water again poured 

 on, time being allowed for it to act, then again decanted, and so on until fresh 

 water does not take up anything. But then finally such weak solutions are obtained 

 that it would be very disadvantageous to evaporate them. This is avoided by pouring 

 the fresh hot water destined for the lixiviation, not onto the fresh mass, but upon a mass 



