Skby. — 0)1 the Production of Artificial Chromes. 361 



and sulpho -arsenide of cobalt. All these, however, are of 

 too dark a colour to permit of their use for ornamental 

 purposes. 



The best way to obtain cupric-sulphide films upon minerals 

 is as follows ; One pound of hyposulphite of soda is dissolved 

 in two pints of water, and then mixed with an aqueous 

 solution of commercial sulphate of copper, consisting of 

 loz. of the salt to eight parts of water. A little more or 

 less of either salt may not matter much, but what is abso- 

 lutely necessary is to have a large excess of the hypo- 

 sulphite, which is known by the thorough discolourization of 

 the cupric compound. If too little is used a brown film of 

 cupreous sulphide forms, which is not sufficiently transparent 

 for the object in view. 



The mineral to be treated is attached to a piece of string 

 and plunged into about ten times its bulk of the mixed 

 solution, to which heat is then applied. Just as it begins to 

 boil the liquid turns green, and cupric sulphide is precipitated, 

 principally as a black incoherent powder ; a small proportion, 

 however, attaches itself to the mineral and to the sides of the 

 containing vessel. About half a minute after the solution 

 begins to boil the mineral should be lifted out for examination. 

 The first colour to show is, in the case of galena, a ruby-red ; 

 then, in turn, as the immersion is prolonged, yellow, emerald, 

 green, blue, black-blue, and so on, in the reverse order of the 

 spectrum as usually stated. 



Should the mineral, by too long immersion, pass beyond 

 the desired colour, an aqueous solution of ammonia will 

 restore the requisite hue by reducing the thickness of the 

 film ; whilst, should it be necessary to remove the film 

 altogether, a strong solution of cold cyanide of potassium will 

 accomplish this at once and leave the specimen itself quite 

 uninjured, this salt having little effect on the common 

 sulphides. 



So far I have dealt only with opaque minerals. In the 

 case of transparent objects it was necessary to find an opaque 

 and lustrous substance capable of being deposited in the same 

 manner as cupric sulphide, and upon whose outer surfaces a 

 film of cupric sulphide could be deposited with the same effect 

 as when these massive sulphides were used. 



After several experiments I was successful in finding this 

 reflecting background— namely, sulphide of nickel, which can 

 be precipitated in thin films from its hyposulphite solution. 

 Two operations are therefore necessary to coat transparent 

 objects such as glass with iridescent cupric sulphide. They 

 must be first enfilmed with sulphide of nickel, as precipitated 

 from its hyposulphite, and afterwards coated with the cupric 

 sulphide by the process already described. 



