362 Transactions. — Chemistry. 



It only remains for me now to inform you what I consider 

 the exact natiire of the fihns which, by their action on hght, 

 afford the very beautiful colours of the specimens in the jars 

 before you. 



These films are pure protosulphide of copper, and are 

 therefore of precisely the same chemical nature as the mineral 

 covelline. 



In chemical and mineralogical works covelline is described 

 as an opaque mineral. This is no doubt true as regards its 

 massive form ; but, as it appears of a purplish colour by reflected 

 light, it should certainly show the complementary colour to 

 this — namely, green — when vievi^ed in thin sections by trans- 

 mitted light. An inspection of the thickest of the films on the 

 glass slides will prove that, like covelline, they reflect a pm'ple 

 or purplish-blue colour. 



That the nature of the films is as I have asserted, is capable 

 of further demonstration by the aid of the electrical instru- 

 ments on the table. Both the films and the covelline, when 

 connected with the poles of the small battery, allow an electric 

 current to pass along them, as can be seen by the vigorous 

 movement of the galvanometer. That cupric-sulphide films 

 are fairly good conductors of electricity is shown by the fact 

 that even the thinnest red-coloured films, when deposited on 

 stibnite, which is itself a non-conductor, conduct the current, 

 and they do this without being at all disrupted or heaved 

 into corrugations, contrary to what I found in 1860 to be the 

 case with films of silver deposited upon glass by organic 

 matters from argentic salts, when subjected to a similar 

 test. 



These artificially-produced films are also of the same 

 nature as those on bornite and peacock copper ore. These 

 natural films are hkewise conductors of electricity, and, 

 like those deposited on glass slides and galena, are quickly 

 removed by cyanide of potassium, the resulting solution con- 

 taining copper and sulpho-cyanogen. 



Before these investigations, I had the opinion, in common 

 with many others, that these natural films were metallic 

 oxides ; but the electric battery applied to them at once dis- 

 abused me of this erroneous belief, and led me to the applica- 

 tion of the cyanide test, by which their real nature was quickly 

 demonstrated. 



The beautiful iridescence, then, which nature has given to 

 certain of the copper ores is clearly due to films which are of 

 the same character as those deposited on the exhibits now 

 before you : but the processes by which each is accomplished 

 are very different— that of nature being by the simple oxidation 

 and removal of the sulpliide of iron from the compound sul- 

 phide, leaving the cupric sulphide behind ; that of man by the 



