178 PROCEEDINGS OP THE AMERICAN ACADEMY 



twenty-four hours than was the case before, because of the warmer 

 weather. Several determinations had to be rejected because the tem- 

 perature rose above zero, and copper was precipitated with the silver. 

 Upon splitting open the fine crystalline plates of silver precipitated 

 in these rejected experiments, a light green precipitate of basic cupric 

 nitrate was found adhering to the inner surface, which could not be 

 removed by continued washing with cold water. The presence of this 

 precipitate explains the admixture of copper with the silver precipi- 

 tated above zero, and points at once to the mechanism of the chemical 

 action. 



When copper is placed in a solution of argentic nitrate, two reactions 

 take place, and the temperature regulates the predominance of one or 

 the other. The chief reaction is the simple one ordinarily written ; it 

 alone is active between 0° and —5°, and it is the chief one even at 

 100°. The secondary reaction, which is active at 100°, but which 

 entirely ceases below 0°, may perhaps be written thus, — assuming 

 that the basic nitrate has the formula usually assigued to it : — 



4 AgN0 3 + 4 Cu + 3 H 2 = 



Cu(N0 3 ) 2 . 3 Cu(OH) 2 + 2 Ag 2 + NO + N0 2 . 



Evidently in this reaction the copper precipitates only one half of 

 its equivalent of silver. It will be remembered that an evolution of 

 nitrous fumes was previously observed, when the temperature rose 

 above the freezing point. 



The argentic nitrate used in the new series was prepared as before, 

 except that even greater precautions were taken to insure its purity, 

 by successive crystallizations and fusions. 



Two samples of copper were used, one from Lake Superior, the 

 other from Germany. For the purification of the former, the sample 

 was dissolved in sulphuric acid, with the addition of nitric acid ; the 

 solution was evaporated to dryness, and the solid heated over asbestos 

 in a porcelain dish, until the fumes of sulphuric acid ceased coming off. 

 The cupric sulphate was now dissolved in water, crystallized twice, 

 and the diluted solution of the last crystals boiled and shaken with a 

 little potassic hydrate for three hours. The solution was now filtered, 

 and the cupric sulphate was crystallized several times from hot water. 

 Finally, the solution of the last pure crystals, strongly acidified by sul- 

 phuric and a little nitric acid, was decomposed by the current of a 

 Bunsen cell, and the chemically pure copper deposited on thick plati- 

 num foil. 



The second example of copper was prepared from German cupric 



