232 Dr J. Davy 07i the Action of 



Having now described the facts which I have observed rela- 

 tive to the action of the mineral acids on copper in these differ- 

 ent circumstances, I have little else to add. The phenomena 

 are evidently of the same class as those which were the subject 

 of my former paper, and equally referable to electro-chemical 

 action. In the first set of experiments, in which atmospheric 

 air was excluded, or very nearly so, scarcely any change was 

 observable, excepting in the instance of the nitric acid, and the 

 change in that case was probably connected with the decompo- 

 sition of a small part of the acid. In the second set of experi- 

 ments, on the contrary, the changes which took place were nu- 

 merous and complicated, owing to the presence of atmospheric 

 air, and the reaction of the combinations formed on each other. 

 And, in the third set, in which the circumstances of the experi- 

 ments were still more various, the effects were produced more 

 rapidly, though less distinctly, and as well in the instance of 

 the sulphuric acid as of the nitric and muriatic. It may appear 

 extraordinary, that the peroxide of copper was formed, and, I 

 may say, deposited in the first set of experiments, and that it 

 was not dissolved by the acids. To what cause the formation 

 and deposition of this oxide was owing, I am at a loss to con- 

 ceive, and I can offer no suggestion in the least satisfactory to 

 myself. It is almost as obscure as an effect which I have ob- 

 served, on immersing a polished bar of copper in a neutral solu- 

 tion of sulphate of copper, when copper in its metallic state, in 

 very minute quantity, is precipitated *. The black oxide not be- 

 ing dissolved when deposited is not surprising, considering that 

 it is an oxide of difficult solubility, even in the strong mineral 

 acids, and much more so when these acids are diluted with water. 

 Why the protoxide of copper should have made its appearance 

 when atmospheric air was admitted in the experiments, and oti- 

 ly then, is probably owing to the action of an electro-chemical 

 cause. Moreover, I may remark, that, when copper is either 

 put into an open fire or left in distilled water, exposed to the 

 action of atmospheric air, the same oxide is formed ; and, in the 

 latter instance, the colouring effect is so brilliant and beautiful, 



• This I conceived might have been occasioned by the sun's rajs ; for it 

 was in making some experiments on their chemical agency, that I first no- 

 ticed the phenomenon ; but my conjecture was not confirmed on repeating 

 the trial in the da^l- wi^^" *iio same precinitation occurred. 



