474 SODIUM. 



sulphuret of calcium, when moistened, is decomposed easily and 

 completely by a single equivalent of carbonic acid. Hence the 

 application of hydrochloric acid to the waste may be made, with 

 the evolution of nothing but sulphuretted hydrogen ; and the 

 deficiency in the quantity of hydrochloric acid may be made up 

 by a supply of carbonic acid, to be applied to the waste, from 

 any other source. The sulphuretted hydrogen is burned, in- 

 stead of sulphur, in the leaden chamber, to reproduce sulphuric 

 acid. 



Many changes have been proposed upon the soda process. 

 Sulphate of iron, produced by the oxidation of iron-pyrites, is a 

 cheap salt, and may be applied to convert chloride of sodium 

 into sulphate of soda, (1) by igniting a mixture of these salts 

 in a reverberatory furnace, when sulphate of soda, peroxide of 

 iron and volatile perchloride of iron are produced : (2) by dis- 

 solving the salts together in water, and allowing the solution to 

 fall to a low temperature, when sulphate of soda crystallizes, 

 and chloride of iron remains in solution (Mr. Phillips); or (3) by 

 concentrating the last solution at the boiling point, when the 

 same decomposition occurs, anhydrous sulphate of soda precipi- 

 tates, and may be raked out of the liquor. Sulphate of magne- 

 sia has also been substituted for sulphate of iron, in these three 

 modes of application. It has been proposed, instead of fur- 

 nacing the sulphate of soda, to decompose it by caustic barytes. 

 Chloride of sodium has also been decomposed by moistening it, 

 and rubbing it in a mortar with 4 or 6 times its weight of litharge, 

 when an oxichloride of lead is formed, and caustic soda liberated. 

 The decomposition of chloride of sodium by the carbonate of 

 ammonia, with formation of bicarbonate of soda, has already 

 been noticed (page 469). It appears, however, that the soda- 

 process first described, which was invented towards the end of 

 last century by Leblanc, is still generally preferred to all others. 



The old sources of carbonate of soda, namely barilla, or the 

 ashes of the salsola soda, which is cultivated on the coasts of 

 the Mediterranean, and kelp, the ashes of sea-weeds, have 

 ceased to be of importance, at least, in England. Barilla con- 

 tains about 18, and kelp about 2 per cent of alkali. 



Bisulphate of soda ; HO, SO 3 + NaO, SO 3 ; 1505.7 or 120.64. 

 This salt is obtained in large crystals on adding an equivalent of 

 oil of vitriol to sulphate of soda, and evaporating the solution 

 till it attains the degree of concentration necessary for crystalliza- 



