l6 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



For this purpose the very pure hydrochloric acid was used whose 

 preparation is described above (p. 12). This acid was neutralized 

 by an especially prepared sample of soda, made from a second por- 

 tion of the Syracuse bicarbonate. The purification was conducted 

 wholly in platinum vessels, of course. After being washed as before, 

 the bicarbonate was dried and heated, to convert it into the normal 

 carbonate. The solution of the latter in water was decanted daily as 

 long as any precipitated residue (calcic carbonate) appeared visible. 

 The sodic carbonate was then recrystallized four times, as the deka- or 

 hepta-hydrate, and once as the anhydrous salt. A concentrated 

 solution of the latter salt was then treated for a considerable time with 

 carbon dioxide under a bell jar until the bicarbonate had formed. 

 The carbon dioxide was made by gently heating sodic bicarbonate, 

 and was well washed with pure water. The pure bicarbonate thus 

 formed was washed with cold water, thoroughly drained, and dissolved 

 in an excess of the pure hydrochloric acid. The resulting salt was 

 twice fractionally recrystallized from water on the steam bath ; and the 

 crystals were washed and drained each time, in order to free them from 

 any possible calcium or magnesium chlorides, which are not easily 

 eliminated from carbonate. Since the atomic weight from this prepa- 

 ration (I,) was identical with that from the preceding preparations, the 

 previous salts, made by precipitation with hydrochloric gas, could have 

 contained no other halogen than chlorine. 



It appears from the identity of all these specimens that sodic 

 chloride is among the substances whose preparation in a pure state is 

 an easy problem. It is further true that the expulsion of water from 

 it upon fusion without loss of halogen is a very simple matter ; more- 

 over, since we succeeded in proving that salt fused in a vacuum 

 possesses the same combining weight as that fused in air, the solid 

 contains neither occluded oxygen nor nitrogen in weighable amounts. 

 This matter is discussed further on page 55. 



Considering these experiments, it is fairly safe to infer that all the 

 various preparations of sodic chloride were quite free from significant 

 quantities of any foreign substance. 



SILVER. 



Recrystallized silver nitrate served as the starting point for the 

 first specimens, and the already purified residues from the early experi- 

 ments were again converted into pure silver for later use. Many 

 samples were prepared from different sources. 



The silver used in all the experiments on sodic chloride was at 

 first purified by precipitation as chloride and then recovered by reduc- 



