426 PRINCIPLES OF CHEMISTRY 



maghesium salts. If the salt be pure, its solution gives -no precipitate 

 with sodium carbonate, Na 2 C0 3 , showing the absence of magnesium 

 salts, because magnesium carbonate, MgCO 3 , is insoluble in water. 

 Rock salt, which is ground for use, generally contains also a considerable 

 admixture of clay and other insoluble impurities. 8 For ordinary use 

 the bulk of the salt obtained .can be employed directly without further 

 purification ; but some salts are purified by solution and crystallisation 

 of tHe solution after standing, in which case the evaporation is not car- 

 ried on to dryness, and the impurities remain in the mother liquor or 

 in the sediment. When perfectly pure salt is required for chemical 

 purposes it is best obtained as follows : a saturated solution of table 

 salt is prepared^ and hydrochloric acid gas is passed through it ; this 

 precipitates the sodium chloride (which is not soluble in a strong solu- 

 tion of hydrochloric acid), while the impurities remain in solution. By 

 repeating the operation and fusing the salt (when adhering hydro- 

 chloric acid is volatilised) a pure salt is obtained, which is again 

 crystallised from its solution by evaporation. 9 



Pure sodium chloride, in well-defined crystals (slowly deposited 

 at the bottom of the liquid) or in compact masses (in which form rock 

 salt is sometimes met with), is a colourless and transparent substance 

 resembling, but more brittle and less hard than, glass. 10 Common 

 salt always crystallises in the cubic system, most frequently in cubes, 

 and more rarely in octahedra. Large transparent cubes of common 

 salt, having edges up to 10 centimetres long, are sometimes found in 

 masses of rock salt. 11 When evaporated in the open the salt often- 



8 The fracture of rock salt generally shows the presence of interlayers of impurities 

 which are sometimes very small in weight, but visible owing to their refraction. In the 

 excellently laid out salt mines of Briansk I counted (1888), if my memory does not 

 deceive me, on an average ten interlayers per metre of thickness, between which the salt 

 was in general very pure, and in places quite transparent. If this be the case, then there 

 would be 850 interlayers for the whole thickness (about 85 metres) of the bed. They 

 probably correspond with the yearly deposition of the salt. In this case the deposition 

 would have extended over more than 300 years. This should be observable at the present 

 day in lakes where the salt is saturated and in course of deposition. 



9 My own investigations, have shown that not only the sulphates, but also the 

 potassium salts, are entirely removed by this method. 



10 According to the determinations of Klodt, the Briansk rock salt withstands a 

 pressure of 840 kilograms per square centimetre, whilst glass withstands 1,700 kilos. 

 In this respect salt is twice as secure as bricks, and therefore immense masses may be 

 extracted from underground workings with perfect safety, without having recourse to 

 brickwork supports, merely taking ^advantage of the properties of the salt itself. 



11 To obtain well-formed crystals, a saturated solution is mixed with ferric chloride, 

 several small crystals of sodium chloride are placed at the bottom, and the solution 

 is allowed to evaporate slowly in a vessel with a loose-fitting cover. Octahedral crystals 

 are obtained by the addition of borax, urea, &c., to the solution. Very fine crystals are 

 formed in a mass of gelatinous silica. 



