SODA ASH. li 



western islands of Scotland and in Ireland. Barilla or kelp, was at the 

 best but a limited source, and Leblanc's process was, therefore, a great 

 improvement, when it enabled us to obtain soda from such a plentiful 

 substance as common or sea salt. 



We deem it sufficient to indicate in outline only the different stages of 

 Leblanc's process, as a full description of them would hardly prove 

 interesting to our readers. 



The first stage consists in converting the chloride of sodium, or com- 

 mon salt, into sulphate of soda, by heating it in a reverberating furnace 

 along with oil of vitrol. Hydrochloric acid is given off during the 

 process. This gas is not allowed to escape into the atmosphere as it 

 once was, but is condensed in an arrangement known as the Coke 

 Tower. The sulphate of soda, which is left in the furnace, is called salt 

 cake. 



The second stage consists in roasting the salt cake of the last operation 

 along with a mixture of chalk and ground coal in a reverberating furnace 

 until it is completely fused. Carbonic oxide gas is given off abun- 

 dantly during the process. The fused mass on being withdrawn from 

 the furnace, is now called hall soda or black ash. 



The third stage consists in dissolving out of this black ash the valu- 

 able soda salts. This is done by a most ingenious application of tepid 

 water, by means of which a large amount of black ash is thoroughly 

 exhausted of its soda salt by a comparatively small amount of water. 

 "What the water does not dissolve is known as soda waste. It consists 

 mainly of oxysulphide of calcium. 



The soda liquor or lye, which is thus obtained, is then evaporated to 

 dryness, and once more calcined along with some sawdust or coal dust, 

 the effect of which is to decompose any sulphide of sodium, and convert 

 it into carbonate of soda. It undergoes another purification by being 

 once more dissolved, evaporated, and calcined. The product of this last 

 operation, on being ground under mill-stones, constitutes the soda ash of 

 commerce. It may be regarded as a mixture composed in chief part of 

 carbonate of soda, and in smaller quantity of caustic soda ; but, besides 

 these, it may contain such impurities as sulphide of sodium, hyposul- 

 phite, and sulphate of soda, particles of sawdust, &c. 



If the relative quantities of carbonate of soda and caustic soda re- 

 mained always the same, in every sample of soda ash, there would be no 

 use for processes for valuing the article ; but, as these frequently vary, 

 there has long been in use a method of estimating the exact value of any 

 sample of soda ash. These methods are applicable to pearl ash as well, 

 and are known under the general name of alkalimetry. 



An alkalimetrical method is based on the well established fact, that 

 a certain known quantity of an acid, such as sulphuric acid, will always 

 neutralise or combine with a fixed definite quantity of alkali, such as 

 soda or potass ; and it is easy to tell, by the use of a little colouring 

 matter, such as litmus, when this neutralising has been effected. The 



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