166 PROCEEDINGS OF THE AMERICAN ACADEMY. 



upon charcoal. The himps thus obtained were dissolved in pure nitric 

 acid and precipitated by electrolysis. The beautiful crystals of elec- 

 trolytic silver were rapidly fused upon cupels of sugar charcoal * in the 

 flame of illuminating gas, and cooled in a reducing atmosphere. Such 

 silver is essentially identical with the mucli more carefully prepared 

 metal used in the final experiments, and gives every evidence of being 

 pure within two or three parts in a hundred thousand. f A solution of 

 argentic nitrate through which over fifty grams of such silver had 

 passed by electrolysis, was found upon suitable treatment to yield only 

 half a milligram of baric sulphate ; hence the silver could not have 

 contained more than two parts of sulphur in a million. 



Preparation of other Materials. — Nitric acid was repeatedly dis- 

 tilled, the last portion of the distillates being used. It is needless to 

 say that it was wholly free from halogens. The sulphuric acid used for 

 analytical purposes was distilled with great care ; that used for desic- 

 cators was boiled with a little aramonic sulphate. The water used in 

 the preliminary determinations was twice distilled in a tin condenser. 

 In order to avoid the introduction of chlorine, carbon dioxide was 

 prepared at first from acid sodic carbonate and sul[)huric acid, later 

 from dilute nitric acid and marble. Nitrogen mixed with argon was 

 prepared by passing a mixture of air and ammonia over red-hot copper. 

 Both gases were very thoroughly washed. 



Method of Analysis. — The necessity of driving out every trace of 

 water from the substance to be analyzed was the precaution upon 

 which most labor was expended. The low boiling points of zincic 

 bromide makes it possible to distil the substance easily in a hard glass 

 tube. Accordingly, for the first series of very crude experiments pure 

 zincic bromide was distilled in a tube provided with bulbs, which were 

 sealed off when filled. In order to obviate the introduction of an error 

 from the additional weight of the atmosphere of carbon dioxide in 

 which the distillation took place, the bulbs were heated to about 120° 

 at the moment of sealing. The bulbs were weighed after scratching 

 them upon each end with a file ; and after the zinc bromide had been 

 dissolved out the glass was weighed alone. The bromine present was 

 weighed as argentic bromide, and the atomic weight of zincic was 

 calculated from the ratio of argentic and zincic bromides. The more 

 trustworthy results ranged from 65.40 to 65.54, with an average of 

 65.47 ; but it was clear that the method admitted of too many possi- 

 bilities of error to yield satisfactory results. 



* These Proceedings, XXIX. 65. t Ibid. 



