TECHNICAL CHEMISTRY. 329 



COMPOSITION OF SALT-CAKE. 



Sodium sulphate, Na a S0 4 96'150 



Calcium sulphate, CaS0 4 0'923 



Sulphuric acid, H,SO 4 0-616 



Sodium chloride, NaCl 1'345 



Ferric oxide, Fe 2 3 0'191 



Water, H 2 0'187 



Insoluble matters 0'130 



Loss .. 0-093 



100-000 



The furnaces such as I have described are termed open 

 roasters, in opposition to the second kind of salt-cake furnace 

 to which the name of close roasters is given. In the first kind 

 the hot air and flames from the fire pass over the salt-cake, 

 and the products of combustion pass along with the acid 

 vapours into the condenser. Hence by this mode of work- 

 ing, a part of the condensed acid is rendered both weak 

 and impure, and much annoyance is caused by the con- 

 densers becoming choked with soot and dust from the fires. 

 For this reason the second method is employed in many 

 works. The pan is built at the side of the roaster instead 

 of being placed in the centre, and the acid from the first 

 part of the decomposition being concentrated and unmixed 

 with air, passes into a condenser where a saturated or fuming 

 aqueous hydrochloric acid is prepared, whilst the gases from 

 the roaster are separated from the products of combustion 

 by enclosing the hearth of the furnace by a fire-brick arch, 

 between which and the top of the furnace the flames and 

 hot air from the fire pass. So that the salt-cake on the 

 hearth is placed in a kind of brick chamber or muffle, being 

 heated from the hearth under which the fire passes and by 

 radiation from the hot arch. Thus no soot or dirt from 

 the fire can be carried into the condensers, and these do not 

 become clogged or choked, and thus a perfect condensation 

 is rendered possible. 



Unfortunately these advantages are not wholly unaccom- 

 panied by drawbacks, which if not important for the manu- 

 facturer, are at least serious to his neighbours. The arch 

 separating the roaster-hearth from the fire gases cannot in 

 practice be kept gas-tight. It is continually cracking from 



