CHLORIDE BERTHOLLET'S LAWS 415 



salt are consumed per head of population, directly in food or for cattle. 

 In those countries where common salt is employed in technical pro- 

 cesses, and especially in England, almost an equal quantity is consumed 

 in the production of substances containing chlorine and sodium, and 

 especially in the manufacture of washing soda, &c., and of chlorine 

 compounds (bleaching powder and hydrochloric acid). 



Although certain lumps of rock salt and crystals of bay salt some- 

 times consist of almost pure sodium chloride, still the ordinary com- 

 mercial salt contains various impurities, the most common of which are 

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

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

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

 Rock salt, which is ground for use, further generally contains a large 

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

 the bulk of the salt obtained is quite suitable without further purifica- 

 tion ; but some salts are purified by solution and crystallisation of 

 the solution after standing, in which case the evaporation is not carried 

 on to the end, and the impurities remain in the mother liquor or 

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

 purposes it is best to proceed 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 the form of well-formed crystals (slowly 

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

 rock salt is sometimes met with), is a colourless and transparent sub- 

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



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, belonging to Mr. Letunoffsky, 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 350 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 I have personally convinced myself that by this method not only the sulphates, 

 but also the potassium salts are entirely removed. 



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

 pressure of 840 kilograms per square centimetre, whilst glass withstands 1700 kilos. 



