206 PROCEEDINGS OP THE AMERICAN ACADEMY 



Finally, 0.3530 gram of purest electrolytic copper lost 0.00005 

 gram on heating to incipient redness in an atmosphere of hydrogen. 

 After dissolving in nitric and sulphuric acids and electrolyzing pre- 

 cisely as usual, the recovered copper weighed 0.35305 gram, indicating 

 a gain of only one tenth of a milligram over the lowest weight previ- 

 ously observed. This gain is not greater than a possible error in 

 weighing; hence, as proof of the accuracy of the electrolytic method 

 and the purity of the materials, the experiment leaves little to be 

 desired. v 



II. Final Determinations. 

 Preparation of Cupric Bromide. 



In preparing cupric bromide for the final series of experiments it was 

 decided to adopt a wholly new method, namely, the action of excess of 

 bromine upon copjier in the presence of water. Pure copper was pre- 

 pared from a new source, the chemically pure sulphate from a noted 

 German house, by the method which has so often been described.* 

 The salt was freed from a possible trace of bismuth and iron by 

 potassic hydrate in very dilute solution, precipitated from the concen- 

 trated filtrate by sulphuric acid, and after many re-crystallizations was 

 finally twice successively decomposed by fractional electrolysis. The 

 resulting copper was a very brilliant and beautiful substance ; after 

 being thoroughly washed and dried at 90°, it showed no trace of oxi- 

 dation on standing three months. 



The problem of the preparation of pure bromine has been admira- 

 bly solved by Stas,t and the method adopted in the present case was 

 largely based upon his, although differing from it in several important 

 particulars. Sixty grams of potassic bromide in dilute solution were 

 completely freed from a trace of iodine by two additions of a small 

 quantity of bromine water, shaking out each time with successive 

 portions of pure carbon disulphide until the two liquids in the separat- 

 ing funnel were alike colorless. After evaporation, the aqueous solu- 

 tion of potassic bromide was mixed with pure dilute sulphuric acid and 

 pure powdered manganese dioxide, and the mixture was subjected to 

 distillation from a glass retort over a water bath, into a glass condenser 

 packed in ice. The manganese dioxide had previously been washed 

 with an aqueous solution of bromine, pure water, hot dilute sulphuric 

 acid, and finally with pure water again until neutral to litmus. 



* These Proceedings, xxii. 346, xxiii. 178; this paper, pag;e 199. 



t Stas's Untersuchungen, p. 158 ; Uebergesetzt von L. Aronstein, Leipzig. 



