OF ARTS AND SCIENCES. 281 



second remained perfectly clear. The excess of baric hydroxide iu 

 the first tube was quickly neutralized with weak hydrochloric acid and 

 phenol phthalein, and the precipitate was filtered off and washed. 

 The absence of barium from this precipitate proved the absence of 

 carbon dioxide from the cupric oxide. 



In order to test the adequacy of the method, 3.4 milligrams of so- 

 dic carbonate were added to the cupric chloride through the funnel 

 tube. This quantity yielded only one tenth the amount of carbon 

 dioxide necessary to account for the difference between the atomic 

 weights of co|)per in question, but it nevertheless produced a heavy 

 white precipitate in both newly filled baric hydroxide tubes. The 

 precipitate when tested showed large quantities of barium ; hence the 

 method was quite competent to decide that the absorption of carbonic 

 acid by cupric oxide was not the error for which search was being 

 made. 



Experiment 57. — Although it seemed very improbable that cupric 

 oxide could contain even traces of a higher oxide after ignition at a red 

 heat, proof of the point was not at hand. Accordingly, four grams of 

 the substance were boiled with pure hydrochloric acid in an apparatus 

 somewhat resembling the last one, in which all the joints were of 

 sealed or ground glass. The vapors were driven through a reversed 

 air cooler into bulbs containing a strong cooled solution of potassic 

 iodide. The very small amount of iodine set free after some time was 

 determined by means of sodic thiosulphate. Tiiis amount corre- 

 sponded to 0.05 milligram of oxygen but even this was undoubtedly 

 due to copper in uncondensed spray. Eight tenths of a milligram of 

 potassic dichroraate, subsequently added to the cupric chloride, at once 

 set free more than double this amount of halogen in the bulbs. The 

 method was hence shown to be adequate for the purpose. The results 

 vrove the absence of an essential amount of any higher oxide or oxi- 

 dized nitrogen from ordinary cupric oxide. 



The Determination of Occluded Gases. 



After so much negative evidence, the only remaining hypothesis 

 which could account for the irregularity of the atomic weight of 

 copper deduced from cupric oxide is that of the occlusion of gases by 

 the substance. This idea is by no means new. As long ago as 1842 

 Erdmann and Marchand* showed that at least a small amount of air 



* J. f. prakt. Clieni., XXVI. 4G1. 



