220 



Mr. V. H. Yeley. The Conditions of the [June 6, 



nitric acid = 27'25), into which, was also placed 0*9445 gram of 

 copper nitrate (prepared by dissolving a few of the discarded copper 

 spheres in dilute nitric acid, evaporating the solution over the water- 

 bath, and twice recrystallising from water). At a temperature of 

 25° C. no gas was evolved for 2' after insertion of the copper sphere; 

 it was then taken out, the liquid agitated mechanically, and the sphere 

 again introduced ; no gas was evolved for 2' 50". A further quantity 

 of 2 grams urea, dissolved in 2 c.c. water, was introduced and the 

 experiment repeated ; no gas was evolved for 7' 30". These experi- 

 ments show that even in the presence of a small quantity of copper 

 nitrate the addition of comparatively small proportions of urea will 

 temporarily prevent the solution of the copper. It also appeared that 

 when once the reaction had set in it could not be stopped, even though 

 there was present in the acid more than sufficient urea to destroy the 

 nitrous acid which might have been formed. For after agitation of 

 the liquid the copper was again unattacked by the acid. If, then, the 

 apparently purely local formation of nitrous acid could be prevented, 

 it might be possible to prevent also any reaction between the copper 

 and nitric acid from taking place. 



To test this point, 1 gram of urea dissolved in I c.c. of water was 

 added to a litre of nitric acid taken from the same sample as that used 

 in the preceding experiments. The little glass dish was more quickly 

 rotated by means of the machine, and a very rapid current of carbonic 

 acid passed in, so that the whole liquid was kept in violent agitation. 

 The copper sphere was introduced, the acid being heated to 25° C. 

 During an interval of one hour no gas was evolved, no blue colour 

 imparted to the acid, no alteration of the metallic surface apparent, 

 and the loss in weight experienced was only 0'0025 gram. Whereas, 

 if all other conditions had remained the same, except that the nitrous 

 acid had been initially present and had been allowed to accumulate, 

 the loss of weight from a sphere exposing a surface of 256"85 mm. 

 would have been about 0*3 gram (calculated from the results given in 

 Table I, the conditions of the experiments in the two cases being 

 practically identical). This experiment might doubtless have been 

 allowed to proceed for a longer time with the same result. 



It shows, I believe, that pure copper will not dissolve in pure dilute 

 nitric acid, for though the proportion of nitric acid to that of urea 

 present was, under these conditions, 272'5 : 1, yet the small quantity 

 of the latter was sufficient to completely stop the reaction. It also 

 appears that the dissolution of copper in nitric acid is dependent 

 solely upon the local production of nitrous acid, effected probably 

 by local electric circuits due to traces of impurities in the copper ; 

 the proportion of nitrous acid initially produced is possibly also 

 increased by a local rise of temperature. As regards this point 

 Dr. Russell writes (loc. cit.) : "The nitrous acid attacks the silver, 



