6l2 PHILOSOPHICAL TRANSACTIONS. [aNNO IJSO. 



we would call the oil of metals. The nitrous acid decomposes this oil, at the 

 same time that it acts on the calx itself, and leaves it also to be acted on by the 

 muriatic acid. When copper is combined with sulphur iu an ore, it is in its 

 metallic form; in dissolving in an acid its oil rises in vapour; or vapours produced 

 by the decomposition of this oil occasion an effervescence. 



All the calces of copper Dr. F. had tried are combined with gas, respirable air, 

 or other vapours, excepting one, which is of a light green colour, brittle, and 

 which breaks smooth like glass; a specimen of it is contained in Dr. Hunter's 

 museum: this dissolves without effervescence; the others all effervesce. A boil- 

 ing heat is necessary to render the solution complete, of which great care is to 

 be taken. If there be any sulphur in the ore, it appears quite clear in lumps; 

 a small portion of it however is destroyed by the nitrous acid. Earthy matters 

 insoluble in acids, if any, and arsenic, appear in a powder at the bottom. If 

 there be any silver, it is mixed with this powder, and is to be extracted by melt- 

 ing it with black flux and litharge, and cupelling in the common way. If there be 

 any gold, it may be taken out of the solution by aether. 



When the copper is combined with nitrous and muriatic acids, it might be 

 thought sufficient to apply the iron immediately; but it is much more convenient 

 to precipitate it from them, and combine it with vitriolic acid, on account 

 of the convenience of washing the precipitate, which is in a more compacted 

 mass. If there be any calcareous earth dissolved, the vitriolic acid will combine 

 with it, and form a white powder, which will be left after the copper is re- 

 dissolved, and must be separated carefully from the solution. After the pre- 

 cipitation of the copper, it is necessary to get rid of the salts perfectly, before 

 applying the vitriolic acid, otherwise part of the copper would be re-dissolved. 

 Vitriolic acid will not dissolve copper in its metallic form, and may be applied to 

 dissolve any iron that may be mixed with the precipitate, as well as to loosen 

 copper, which sometimes adheres to the iron. The solution of the iron must 

 be carefully washed off from the copper. 



There is a criterion by which we may judge certainly if any of the copper be 

 lost. Let all the washings and every thing, except the copper, be put into a 

 vessel together; pour in solution of fixed alkali, till no further precipitation takes 

 place; let the precipitate subside, and pour off the liquor; apply to the precipi- 

 tate solution of volatile alkali, sold by the name of spirit of sal ammoniac; 

 shake them together, and let them stand for an hour; if the solution of the 

 alkali acquires a purplish blue colour, the process is imperfect, if it does not, it is 

 perfect. If the process be imperfect, which is always for want of care in the 

 decantations, pour in as much vitriolic acid as will dissolve the whole precipitate, 

 apply iron to the solution, and the remaining copper will be procured. 



