450 PRINCIPLES OF CHEMISTRY 



preparation of soda ash, as will afterwards be described. For the present 

 we will only turn our attention to the hydrochloric acid evolved in B 

 andC. 



The hydrochloric acid gas evolved is subjected to condensation by 

 dissolving it in water. 32 If the apparatus in which the decomposition 

 is accomplished were hermetically closed, and only presented one outlet, 

 then the escape of the hydrochloric acid would only proceed through 

 the escape pipe intended for this purpose. But as it is impossible to 

 construct a perfectly hermetically closed furnace of this kind, it is 

 necessary to increase the draught by artificial means, or to oblige the 

 hydrochloric acid gas to pass through those arrangements in which it is 

 to. be condensed. This is done by connecting the ends of the tubes 

 through which the hydrochloric acid gas escapes from the furnace with high 

 chimneys, where a strong draught is set up from the combustion of the 

 fuel. This causes a current of hydrochloric acid gas to pass through the 

 absorbing apparatus in a definite direction. Here it encounters a cur- 

 rent of water flowing in the opposite direction, by which it is absorbed. 

 It is not customary to cause the acid to pass through the water, but 

 only to bring it into contact with the surface of the water. The absorp- 

 tion apparatus consists of large earthenware vessels having four orifices, 

 two above and two lateral ones in the wide central portion of each 

 vessel. The upper orifices serve for connecting the vessels together, 

 and the hydrochloric acid gas escaping from the furnace passes through 

 these tubes. The water for absorbing the acid enters at the upper, and 



52 AB in works which treat common salt in order to obtain sodium sulphate, the 

 hydrochloric acid is sometimes held to be of no value, it might be allowed to escape with 

 the waste furnace gases into the atmosphere, which would greatly injure the air of 

 the neighbourhood and destroy all vegetation. In all countries, therefore, there are laws 

 forbidding the factories to proceed in this manner, and requiring the absorption of the 

 hydrochloric acid by water at the works themselves, and not permitting the solution to 

 be run into rivers and streams, whose waters it would spoil. It may be remarked that 

 the absorption of hydrochloric acid presents no particular difficulties (the absorption of 

 sulphurous acid is much mor,e difficult) because hydrochloric acid has a great affinity for 

 water and gives a hydrate which boils above 100. Hence, even steam and hot water, as 

 well as weaker solutions, cfin be used for absorbing the acid. However, Warder (1888) 

 showed that weak solutions of composition H 2 O + nHCl when boiled (the residue will be 

 almost HC1,8H 2 O) evolve (not water but) a solution. of the composition H 2 O + 445n 4 HCl ; 

 for example, on distilling HCl,10HoO, HC1,23H 2 O is first obtained in the distillate. As 

 the strength of the residue becomes greater, so also does that of the distillate, and there- 

 fore in order to completely absorb hydrochloric acid it is necessary in the end to have 

 recourse to water. 



As in Russia the manufacture of sodium sulphate from 'sodium chloride has not yet 

 been sufficiently developed, and as hydrochloric acid is required for many technical pur- 

 poses (for instance, for the preparation of zinc chloride, which is employed for soaking 

 railway sleepers), therefore salt is often treated mainly for the manufacture of hydro- 

 chloric acid/ 



