There are two kinds of _preparation_; the one termed mechanical, from
the means employed, and the results obtained, consists in processes for
breaking and grinding the ores, and for washing them so as to separate
the vein-stones, gangues, or other mixed earthy matters, in order to
insulate or concentrate the metallic parts.
Another kind of preparation, called chemical, has for its object to
separate, by means of fire, various volatile substances combined in the
ores, and which it is requisite to clear away, at least in a certain
degree, before trying to extract the metals they may contain.
Lastly, an indispensable operation in several circumstances, is to
discover, by simple and cheap methods, called _assays_, the quantity of
metal contained in the different species of ores to be treated.