872 



HYDROCHLORIC ACID 



mences ; and then continued moderate till the evolution of gas diminishes, when it 

 must be heated somewhat strongly to finish the decomposition. The iron door is now 

 removed, to extract the sulphate of soda, and to recommence another operation. This 

 sulphate ought to be white and uniform, exhibiting in its fracture no undecomposed 

 sea-salt. 



In preparing hydrochloric acid from a mixture of common salt and sulphuric acid, 

 the nature of the reaction varies with the proportions of raw materials employed. 

 This is a point which demands the attention of the manufacturer. If two molecules 

 of sulphuric acid be used with a single molecule of salt (or 98 parts by weight of acid 

 to 78'5 of salt) the decomposition is completely at a moderate temperature, and bisul- 

 phato (acid sulphate) of soda is produced, whilst hydrochloric acid is evolved. But if 

 two molecules of salt (or 117 parts by weight) be employed, with the same quantity of 

 acid as before, the decomposition takes place in two stages, the final residue con- 

 sisting wholly of neutral sulphate of soda ; but in this case a much higher temperature 

 is required to effect the decomposition. Hence the former method yields the hydro- 

 chloric acid with greater facility, and with less expenditure of fuel, whilst the latter pro- 

 cess requires a higher temperature, but consumes only half the quantity of oil of 

 vitriol. 



Large quantities of hydrochloric acid are produced in the first stage of the manu- 

 facture of soda-ash. Formerly the acid-vapours were allowed to escape into the atmo- 

 sphere, and caused great damage to the surrounding vegetation ; but they are now 

 carefully condensed in large towers or scrubbers. The weak solution of acid thus ob- 

 tained may be afterwards concentrated. Mr. Clapham's process of obtaining a strong 

 acid in this way consists in pumping up the weak acid into the condensing towers, 

 where it becomes strengthened by fresh condensation of vapour. 



Several processes have been suggested for obtaining hydrochloric acid without the 

 use of sulphuric acid, but none of them can be said to be commercially successful. 

 For example, hydrochloric acid may be generated by heating common salt with sul- 

 phate of magnesia, with pyrites, or with weathered sulphate of iron ; the acid may also 

 be obtained by decomposing chloride of calcium by the action of steam at a high tem- 

 perature, or by heating chloride of magnesium. 



Table of Hydrochloric Acid, by Dr. Ure. 



The muriatic acid of commerce has usually a yellowish tinge, due to the presence of 

 chloride of iron ; but when chemically pure, it is colourless. It fumes strongly in the 



