386 



LECTURES TO SCIENCE TEACHERS. 



of labour and in the production of a material which 

 possesses a more constant composition. 



The following Table gives the charges, theoretical and 

 practical, of salt-cake, coal, and limestone used in various 

 works : 



BLACK-ASH CHARGES. 

 Theoretical, Xa 2 S0 4 -f CaC0 3 



No. 5, Tyne charge 



Salt-cake 196 196 



Limestone 252 138 '02 



Coal-dust 126 66 '25 



Here you have a table of analyses of black-ash. You see 

 that it contains a large number of other salts besides car- 

 bonate of soda and mono-sulphide of calcium, though these 

 two constitute its main ingredients. In practice a large 

 excess of lime is used, and this gives rise, as we shall see, to 

 caustic soda in the black-ash. 



