OP ARTS AND SCIENCES. 197 



I. Preliminary Work. 



Preparation and Analysis of Hydrobromic Acid. 



The next step taken was the preparation of hydrobromic acid pure 

 enough to serve as the basis of research. The acid was made from 

 pure potassic bromide and sulphuric acid, according to the method 

 recommended by Dr. Squibb.* It was then purified by repeated frac- 

 tional distillation, at first in a large platinum retort, and afterward in 

 one of glass ; it having been ascertained that the gold used as a solder 

 in the former apparatus was attacked during the process. It was sub- 

 sequently found that the action of the acid even upon glass was not 

 imperceptible, but as the same retort was used in all later distillations, 

 and as the glass was of good quality, this unavoidable cause of im- 

 purity was reduced to a minimum. 



The acid was distilled in all ten times, — seven times over potassic 

 bromide, twice over silver bromide, and once alone, — each time the 

 first and last portions of the distillate being rejected. A specimen of 

 the wholly colorless product was preserved for over a year without 

 perceptible change. 



The necessity of testing the purity of this hydrobromic acid led at 

 once to the next step of the investigation, the preparation of chemically 

 pure silver. Fused argentic nitrate was first mado from pure silver 

 which had been reduced from the chloride by milk sugar and potash. 

 This salt was then dissolved in a large amount of water, and again 

 decomposed by the gradual addition of very dilute hydrochloric acid in 

 slight excess, the precipitate being thoroughly agitated and washed by 

 decantation with dilute hydrochloric acid and hot and cold water. 

 After drying and pulverizing, the -pure argentic chloride was digested 

 for six hours with aqua regia, and then again washed with the purest 

 hot and cold water until the filtrate gave no test for chlorine. 



The final reduction of the cream-colored product was accomplished 

 as before, by warming it with a dilute solution of the purest caustic 

 potash and milk sugar, which had previously been filtered through a 

 Gooch crucible. Careful testing showed the completion of the re- 

 action, and the resulting pure silver, having been thoroughly washed 

 with dilute sulphuric acid and a very large amount of pure hot water 

 until perfectly clean, was collected and dried upon a quantitative filter. 

 Small portions of the powder were successively supported upon a 



* These Proceedings, xvii. 30. 



