75 



aud I shall conclude by describing two of them, as they are the 

 processes generally adopted, being much shorter than the one already 

 described. An assay by either of them may be conducted in about 

 half-an-hour. 



Great attention has lately been paid to the rapid determination of 

 copper by means of standard solutions, and we are indebted to Messrs. 

 Pellouze and Parkes for these valuable methods. 



Pellouze's process depends upon the decolorization of an ammo- 

 niaoal solution of copper, by means of sulphuret of sodium of a known 

 strength. To determine the strength of this solution, a given 

 quantity is dissolved in water, and about twenty grains of pure copper 

 are dissolved in acid, to which ammonia is added in excess. The 

 liquor is boiled in a flask, and the solution of sulphide of sodium is 

 gradually added thereto, from a burette (a graduated measure divided 

 into a hundred equal parts), until the solution becomes colourless, and 

 no more precipitate is formed. We obtain by this means a solution of 

 a known strength ; e.g., supposing we have used fifty measures, by a 

 simple proportion we find that every measure corresponds to 0.4 grains 

 of pure copper. 



To determine the amount of copper then in an ore, if no other 

 metal be present but iron, we adopt the method already described for 

 separating it, add the sulphuret of sodium until no more precipitate is 

 formed, and multiply the number of divisions used by 0.4, being the 

 amount of copper to which each measure corresponds. 



Parke's process is even more easily conducted than that of Pellouze, 

 as the ammoniacal solution of copper does not require boiling, and 

 depends upon its decolorization by means of cyanide of potassium. In 

 this instance no precipitate is formed, but a change of colour takes 

 place. The graduated or standard solution of cyanide of potassium is 

 prepared in exactly the same manner as the sulphuret of sodium, by 

 dissolving a quantity in water, and observing how many measures are 

 required to decolorize twenty grains of pure copper ; and employing 

 the same process (upon fifty or a hundred grains of the ore,) after the 

 iron has been separated. The cyanide of potassium used must be per- 

 fectly pure, and free from ferrocyanide or sulphide of potassium, as both 

 these substances are exceedingly detrimental, aud would completely 

 vitiate the results obtained, 



