440 



PKINCIPLES 



CHEMISTRY 



seen. Hence it may be affirmed that sulphuric acid does not displace 

 hydrochloric acid because of an especially high degree of affinity, but 

 that this reaction is only carried on to the end because the sulphuric- 

 acid is not volatile whilst the hydrochloric acid which is formed is 

 volatile. 



The preparation of hydrochloric acid in the laboratory and on a 

 large scale is based upon these data. In the first instance, an excess- 

 of sulphuric acid is employed in order that the reaction may proceed 

 easily and at a low temperature, whilst on a large scale, when it is 

 necessary to economise every material, equivalent quantities are taken 

 in order to obtain the normal salt Na. 2 SO 4 and not the acid salt (p. 422), 

 which would require twice as much acid. It may be remarked that 

 dry hydrochloric acid is a gas which is very soluble in water. It is 

 most frequently used in practice in this state of solution under the name 

 of muriatic acid. 31 



PlG. 65. Section of a salt eake furnace. B, pan in \vhic 

 are first mixed and heated. C, muffle for the ultimate 



sulphuric acid 



In chemical works the decomposition of sodium chloride by means of 

 sulphuric acid is carried on on a very large scale, chiefly with a view to 

 the preparation of normal sodium sulphate, the hydrochloric acid being 

 a bye-product. The furnace employed is termed a salt cake furnace. 

 It is represented in fig. 65, and consists of the following two parts : the 



31 The apparatus shown in fig. 46 (p. 250) is generally employed for tin- pivp 

 of small quantities of hydrochloric acid. Common salt is plaet-d in the retort ; tin 

 generally previously fused, as it otherwise froths and boils over the appara t u >. \V] 



