280 PROCEEDINGS OF THE AMERICAN ACADEMY 



Experiments 54 and 55. — The idea that some cupric nitrate might 

 remain uudecomposed in cupric oxide is evidently not a new one, for 

 almost every experimenter upon the subject tested for nitric acid in 

 the vpater formed during the reduction. It seemed possible, however, 

 that the nitric acid might be reduced as well as the cupric oxide, and 

 hence that the test might not be a sufficient criterion. Accordingly, 

 five grams of cupric oxide were dissolved in very pure dilute sulphuric 

 aiud, and several successive portions of water were distilled off from 

 the mixture. After evaporation to small bulk in a platinum dish, the 

 distillates were found to be neutral to meihyl orange and acid to 

 phenol phthaleio. When the amount of standard alkali necessary 

 to neutralize this acidity had been determined, hydrochloric acid was 

 added and the sulphuric acid present was estimated as baric sulphate. 

 Since the weight of baric sulphate (3.7 mg.) was nearly equivalent 

 to the amount of alkali used (0.39 c. c. of a decinormal solution), 

 since no nitric acid was found in the filtrate by the most sensitive 

 tests, and since a very perceptible amount of copper was found there, 

 it may be reasonably concluded that cupric sulphate had been carried 

 over mechanically in fine drops. A second experiment yielded like 

 results. Hence, so far as this test was concerned, no nitric acid was 

 to be found in ciipric oxide. 



Experiment 56. — The tendency of various oxides to hold carbonic 

 acid even at high temperatures is well known. Thudichum * has 

 observed that, in order wholly to free cupric oxide from this impurity, 

 it is necessary to ignite it in a vacuum. In order to test this point 

 so far as it concerns the present investigation, the following experi- 

 ment was made. After a small amount of nitric acid had been added 

 to twelve grams of cupric oxide, the substance was gradually brought 

 to redness and maintained at this temperature for ninety minutes. 

 The material was then rapidly transferred to a flask provided with a 

 stoppered funnel and two bent tubes. One of the latter was connected 

 with two test tubes containing a clear solution of baric hydroxide, and 

 the other permitted a gentle current of air to be blown through the 

 entire apparatus. Before either baric hydroxide or cupric oxide were 

 added, all the tubes and flasks were of course freed from carbon 

 dioxide. Finally, dilute boiled hydrochloric acid was run in at the 

 funnel tube, the cupric oxide was dissolved, and the solution was 

 gradually brought to boiling. A slight bluish precipitate of cupric 

 hydroxide appeared after some time in the first test tube, while the 



* Chem. Soc. Journ., 1876, [2], p. 364. 



