14 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



The source of the materials having been given above, it remains 

 to record the details of their preparation. 



In the first place, a large quantity of Merck's sodic chloride, which 

 had been purified from German rock salt, was thrice reprecipitated by- 

 hydrochloric acid. This salt is designated "sample A" below, and 

 was used in preliminary experiments. It was further crystallized from 

 water (B) ; some of this was fused with ammonic chloride and am- 

 nionic chlorplatinate (C) ; a portion was then precipitated with hydro- 

 chloric acid (D) ; and finally some was again fractionated from water 

 (E). None of these operations seemed to have any essential effect 

 upon the combining weight of the salt ; hence, even Sample A must 

 have been practically pure. 



The very pure sodic sulphate was easily converted into chloride 

 by successive precipitation from solution by means of gaseous hydro- 

 chloric acid. The sulphate forms an especially fortunate source of 

 pure sodium, since none of the other alkaline sulphates yield crystals of 

 the same degree of hydration or the same crystal form. Hence sodic 

 sulphate which has been many times recrystallized must be extremely 

 pure. The ease with which a constant transition temperature may be 

 obtained is evidence of this. 



A specimen of sodic sulphate, which had been shown to be pure 

 by its transition temperature, was twice further recrystallized before 

 adding hydrochloric acid. Four precipitations by this gas, with wash- 

 ings and drainings, removed practically all of the sulphuric acid. 

 Nevertheless, when the mother liquor was concentrated by evapora- 

 tion and tested, an extremely small trace of sulphate was found by 

 excess of baric salt to persist until the tenth precipitation. There 

 was no trace of visible baric sulphate on testing the mother liquor, 

 even in the nephelometer, after the eleventh precipitation, but the 

 salt was precipitated once more for certainty. This salt, thus twelve 

 times separated by hydrochloric acid, was designated F. It seems 

 highly probable that since so large an amount of sulphuric acid was 

 eliminated from it by this treatment, all other impurities must have 

 been separated. 



For another specimen of salt, a number of fine, large, colorless 

 transparent crystals of halite, from near Stassfurt, Germany, were 

 dissolved in pure water. After the addition of a little sodic hydroxide, 

 boiling, and standing, the solution was filtered and precipitated with 

 hydrochloric gas. A solution of the latter salt in water stood several 

 days. The clear decanted portion was precipitated twice more, frac- 

 tionally crystallized from water, and dried over sodic hydroxide in a 

 vacuum desiccator. This specimen was called sample G. 



