and Manufacture of Sulphate and Muriate of Potash, 97 



introduced through the roof of the furnace, so as to throw a current of 

 heated steam over the whole width of the hearth ; these pipes are con- 

 nected with a steam-boiler by a series of fire-clay tubes, kept red 

 hot. The sulphate, broken into pieces of about half an inch in dia- 

 meter, is spread over the lining of carbonate of magnesia on the 

 hearth of the furnace, and brought to a high red or low white heat. 

 A current of steam is then admitted from the boiler, through the red 

 hot tubes, upon the charge. 



The acid of the sulphate is carried off by the steam ; and when I 

 wish to condense it, the acid vapours are conveyed, along with the 

 gases of ihe fire, into a leaden chamber, to be combined into sulphu- 

 ric acid by the usual means. The quantity of steam thrown upon 

 the charge is kept at the point which produces the most rapid evolu- 

 tion of acid, and the charge is stirred occasionally, so as to expose 

 fresh surfaces to the action of the steam. As the contact of deoxi- 

 dizing gases with the sulphate is injurious, I admit, if necessary, by 

 suitable openings above the fuel, such an excess of air as will render 

 the atmosphere in the furnace oxidizing. The sulphate of strontia 

 requires a higher heat than the sulphate of lime for its decomposi- 

 tion, and the sulphate of baryta still higher than the sulphate of 

 strontia. 



When the sulphate of baryta is partly decomposed, the mass melts, 

 and becomes more fusible as the decomposition proceeds. I judge 

 of the progress of the operation by testing a portion of the charge 

 from time to time : when it dissolves altogether, or nearly so, in di- 

 lute nitric acid, I withdraw the charge, which now consists chiefly of 

 the hydrate of baryta or strontia. To obtain muriatic acid, and the 

 hydrates of baryta or strontia, or caustic lime, from the muriates of 

 these bases, I employ the same process as that above described for 

 the decomposition of the sulphate of baryta. 



The sulphates of potash and soda may to some extent be de- 

 composed, by being subjected, at a high temperature, to the action 

 of a current of steam, in the manner directed for the decomposition 

 of the sulphate of baryta. But owing probably to the volatile na- 

 ture of the bases of these salts at a high temperature, no large pro- 

 portion of them can thus be obtained in a free state. To aid, there- 

 fore, the decomposing action of the steam, I employ some substance 

 capable, when mixed with these sulphates, highly heated and ex- 

 posed to steam, of forming a combination with their alkaline bases, 

 which shall yet, when cold, give up the alkali to the action either of 

 water or of water and carbonic acid. 



Of the large class of substances possessing these properties, which 

 for convenience I will call combining substances, I prefer to use 

 either alumina or the subphosphate of alumina. The alumina is 

 prepared by strongly igniting the sulphate of alumina, or by any 

 other well-known process. The subphosphate of alumina is pre- 

 pared (as directed in chemical works) by mixing solutions of the 



VOL XLVI. NO. XCI. — JAN. 1849. G 



