THE PREPARATION OF MATERIALS. 33 



The preparation of pure ammonic bromide was very easy. Ammonia 

 essentially free from carbon compounds was redistilled into water in a 

 platinum dish. Into this was dropped the purest bromine, which fell 

 through the liquid into a small porcelain crucible resting on the bottom 

 of the dish. In this way the bromine itself could not come into contact 

 with platinum and attack it. The reaction took place very rapidly, yield- 

 ing ammonic bromide which could have contained no non-volatile ingre- 

 dients, because all its constituents had just been distilled. 



The preparation of pure potassic hydroxide was less easy to devise, 

 because more original ; but the execution was almost as easy. The 

 problem was solved by the use of an electrolytic process which will be 

 described in greater detail elsewhere. In brief, a saturated solution of 

 potassic oxalate (sample I) was electrolyzed between a pure mercury 

 cathode and a platinum anode in a porcelain dish cooled with ice, the 

 current from four storage cells being used. When the amalgam became 

 solid, the current was stopped, the solution decanted, and the amalgam 

 washed, being triturated with an agate pestle under water, till no test 

 for oxalate could be obtained. The pure amalgam was transferred to a 

 platinum dish, covered with water, and made the anode of a dense current 

 until only a little of the potassium amalgam remained undecomposed. 

 Thus pure potassic hydroxide was formed. This solution was poured into 

 the ammonic bromide solution, the mixture was evaporated and crystal- 

 lized, and the salt thus obtained used for analyses 14, 15, and 16. 



The bromide thus obtained must have been very pure. During the 

 electrolysis, the high concentration of the potassic oxalate 1 would tend 

 to allow the deposition of potassium only. 2 At any rate, metals less easy 

 to deionize could hardly have been set free. During the later decomposi- 

 tion of water by the amalgam, all metals with less tendency to ionize 

 would have remained in the mercury, because some of the amalgam was 

 left undecomposed. Any trace of iodine which the bromine may have 

 contained must have been driven off by the fusion just before analysis, 

 this fusion being prolonged in order to expel ammonic bromide. The 

 results of the analyses of this sample indicate its essential identity with 

 the two preceding preparations. Since there was no known source of 

 impurity in any one of the preparations, this was not surprising. 



Although our consistent employment of insoluble vessels rendered the 

 presence of colloidal silica unlikely, an effort was made to test for this 

 impurity of sample II by volatilizing 3 grams of it in a stream of 

 nitrogen. Unfortunately, the high temperature of the electric furnace 

 caused the platinum boat containing the salt to weld to the foil which 



Bunge, Berichte, 9, 78 (1876). 

 2 Berthelot, Ann. Chim. Phys. [5], 18, 433 (1879). 



