The fineness of each needle being marked on it, that needle whose colour seems to come nearest the colour of the metalline streak on the Touch-stone, is rubbed on the stone by the side of that streak. This needle likewise leaves a mark; and if there appear to be no difference between the two metalline streaks, the metalline mass is judged to be of the same fineness as the needle thus compared with it. If the eye discovers a sensible difference, another needle is sought for whose colour may come nearer to that of the metal to be tried. But though a man be ever so well versed in judging thus of the fineness of Gold by the eye only, he can never be perfectly and accurately sure of it by this means alone. If such certainty be required, recourse must be had to the parting assay; and yet when you have gone through it, there always remains a small quantity of the metal, which should have been dissolved, and yet escaped the action of the solvent. For example, if you make use of _aqua regis_, the Silver that remains after the operation still contains a little Gold; and, if you make use of _aqua fortis_, the Gold that remains after the operation still contains a little Silver. And therefore if you resolve to carry the separation of these two metals still further by solvents, it will be necessary, after you have gone through one parting process, to perform a second the contrary way. For example, if you begin with _aqua fortis_, then, after it has dissolved all the Silver in the metalline mass that it is capable of taking up, dissolve the remaining Gold in _aqua regis_: by which means you will separate the small portion of Silver left in it by the _aqua fortis_. The contrary is to be done if you made use of _aqua regis_ first. CHAP. III. _Of_ COPPER. PROCESS I. _To separate Copper from its Ore._