254 A HISTORY OF THE SODA MANUFACTURE. 



remains in the black ash or rough soda obtained, less or more of unde- 

 composed sulphate of soda, and there is produced less or more of 

 soluble sulphide of calcium. 



The true composition of black ash or rough soda, also the rationale 

 of its formation, are not settled questions. It has been held by Dumas 

 and other eminent French chemists, when explaining the reactions 

 which occur in Le Blanc's masterly process, that a peculiar compound, 

 insoluble in water, consisting of sulphide and oxide of calcium (called 

 by them oxy-sulphide of calcium), is formed during the decomposition 

 of sulphate of soda by carbon in the presence of lime ; also that it is 

 essential that sulphide of calcium should be thus combined with oxide 

 of calcium to destroy the supposed solubility of the former in water, 

 and thus prevent the formation of sulphide of sodium at the expense 

 of carbonate of soda. And they assumed that, in consequence of the 

 necessity to form this supposed compound, it was needful to employ 

 more than one equivalent of lime for each equivalent of sulphate of 

 soda used for the production of rough soda. 



According to this theory, it is essential that the supposed insoluble 

 compound should continue to exist in its integrity, so long as it is 

 exposed to the reaction of water or of solution of carbonate of soda. 

 Now I pointed out in the year 1838, in the specification of a patent 

 obtained by me for " Improvements connected with the Soda Manufac- 

 ture," that the undissolved residuum remaining from the lixiviation of 

 black ash with water consisted almost entirely of mono-sulphide of 

 calcium and carbonate of lime. In fact, when the lixiviation is effected 

 with a large proportion of water so as to produce a weak solution, the 

 oxide of calcium may be converted entirely into carbonate of lime, 

 which is then found mixed with the mono-sulphide of calcium ; and 

 yet this conversion of oxide of calcium into carbonate of lime, and 

 consequent disintegration of the supposed insoluble compound, is 

 effected, and the supposed soluble sulphide of calcium is set at liberty, 

 in the presence of solution of carbonate of soda ; and this takes place 

 without occasioning the production of sulphide of sodium in solution. 

 The fact is, that mono-sulphide of calcium is perfectly insoluble in 

 water ; and it is only to the extent to which poly-sulphide of calcium 

 is formed, that solution takes place, and consequent production of 

 sulphide of sodium. The real advantage obtained by the use of an 

 extra proportion of lime arises from this affording a larger amount of 

 surface for re-action, and thus expediting the fluxing operation, and 

 thereby preventing the formation of poly -sulphide of calcium. 



In connection with this view of the subject, I pointed out that, 

 when well-made black ash is digested in alcohol, there is no caustic 

 soda dissolved, although the same black ash yields caustic soda abun- 

 dantly by lixiviation with water. From this I inferred that caustic 

 soda did not exist in the black ash ready formed, and that it was pro- 

 duced, during lixiviation with water, by the re-action of caustic lime 



