520 PRINCIPLES OF CHEMISTRY 



and the clear solution or alkali above it contains the easily-soluble 

 sodium hydroxide formed in the reaction. 25 After the necessary 

 quantity of lime has been added, the solution is allowed to stand, and 

 is then decanted off and evaporated in cast or wrought iron boilers, 

 or in silver pans if a perfectly-pure product is required. 26 The 

 evaporation cannot be conducted in china, glass, or similar vessels, 

 because caustic soda eats into glass and china, although but slightly. 

 The solution does not crystallise on evaporation, because the solubility 

 of caustic soda when heated is very great, but crystals containing 

 water of crystallisation may be obtained by cooling. If the evapora- 

 tion of the alkali be conducted until the specific gravity reaches 1'38, 

 and the liquid is then cooled to 0, transparent crystals appear contain- 



25 As long as part of the undecomposed sodium carbonate remains in solution, excess 

 of acid added to the solution disengages carbonic anhydride, and the solution after dilu- 

 tion gives a white precipitate with a barium salt soluble in acids, showing the presence of 

 a carbonate in solution (if there be sulphate present, it also forms a white precipitate, 

 but this is insoluble in acids). For the decomposition of sodium carbonate, milk of lime 

 that is, slaked lime suspended in water is employed. Formerly pure sodium hydrox- 

 ide was prepared (according to Berthollet) by dissolving the impure substance in alcohol 

 (sodium carbonate and sulphate are not soluble), biit now that metallic sodium has become 

 cheap and is purified by distillation, pure caustic soda is prepared by acting on a small 

 quantity of water with sodium. By allowing strong solutions to crystallise (in the cold) 

 completely pure sodium hydroxide may also be obtained (Note 27). 



In alkali works where the Leblanc process is used, caustic soda is prepared directly from 

 the alkali remaining in the mother liquors after the separation of the sodium carbonate 

 evaporation (Note 14). If excess of lime and charcoal has been used, much sodium 

 hydroxide may be obtained. After the removal as far as possible of the sodium carbon- 

 ate, a red liquid (from iron oxide) is left, containing sodium hydroxide mixed with com- 

 pounds of sulphur and of cyanogen (formed in the Leblanc furnaces, see p. 225, and 

 Chapter IX.) and also containing iron. This red alkali is evaporated and air is blown 

 through it, which oxidises the impurities (for this purpose sometimes sodium nitrate is 

 added, or bleaching powder, &c.) and leaves fused caustic soda. The fused mass is allowed 

 to settle in order to separate the ferruginous precipitate, and poured into iron barrels, where 

 the sodium hydroxide solidifies. Such caustic soda contains about 10 p.c. of water in 

 excess and some saline impurities, but when properly manufactured is almost free from 

 carbonate and from iron. 



26 Lb'wig gave a method of preparing sodium hydroxide from sodium carbonate by 

 heating it to a dull red heat with an excess of ferric oxide. Carbonic anhydride is given 

 off, and warm water extracts the caustic soda from the remaining mass. This reaction, 

 as experiment shows, proceeds very easily, and is an example of contact action similar 

 to the influence of ferric oxide on the decomposition of potassium chlorate. The. 

 reason of this may be that a small quantity of the sodium carbonate enters into double 

 decomposition with the ferric oxide, and the ferric carbonate produced is decomposed into 

 carbonic anhydride and ferric oxide, the action of which is renewed. Similar explana- 

 tions expressing the motive of a reaction really add but little to that elementary concep- 

 tion of contact which, according to my opinion, consists in the change of motion of the 

 atoms in the molecules under the influence of the substance in contact. In order to 

 represent this clearly it is sufficient, for instance, to imagine that in the sodium carbonate 

 the elements CO.> move in a circle round the elements Na.X), but at the points of contact 

 with Fe 2 O 3 the motion becomes elliptic with a long axis, and at some distance from NaoO 

 the elements of CO 2 are parted, not having the faculty of attaching themselves to Fe^O-. 



