334 LECTURES TO SCIENCE TEACHERS. 



when we come to consider the mode of formation of the 

 many distinct compounds which make their appearance in 

 the course of the reaction. I can here only attempt to give 

 an explanation of the chief features of the case. 



The first chemical change which the salt-cake (Na 2 SO 4 ) 

 undergoes in its passage to carbonate of soda is the reduc- 

 tion to sodium sulphide (Na 2 S), by heating it with slack, or 

 powdered coal ; thus : 



Na 2 S0 4 + C 4 = Na 2 S + 400. 



The second change which occurs is the conversion of the 

 sodium sulphide into sodium carbonate (carbonate of soda), 

 by heating it with chalk or limestone (calcium carbonate). 

 The reaction which then takes place is represented by the 

 equation 



Na,S + CaC0 3 = Na,C0 3 + CaS. 



FIG. 12. 



In practice these two reactions are carried on at the 

 same time, a mixture of about 15 parts by weight of salt- 

 cake, 16 parts of limestone, and 6 of coal, being heated 

 in a reverberatory furnace, termed a balling-furnace, an 

 elevation and section of which you see in Figs. 12 and 13. 

 After exposure to the reducing flame of this furnace for 

 two hours the charge consisting of 4| cwt. is fluxed and fully 

 decomposed, and then the liquid mass is scraped out into 



FIG. 13. 



iron barrows or trucks and allowed to cool, and in this state 

 is known as black-ash ball so called from the colour of the 

 mass. 



