this case it is obvious that the water is the solvent. The function of
the acid is to convert the lead into a soluble compound.

~Fluxes~ may act as true solvents. Fused carbonate of soda dissolves
baric carbonate, and perhaps in many slags true solution occurs; but in
the great majority of cases a flux is a solid reagent added for the
purpose of forming a fusible _compound_ with the earthy or stony
minerals of the ore. Few of the minerals which occur in the gangue of an
ore are fusible; and still fewer are sufficiently fusible for the
purposes of the assayer, consequently the subject is one of importance,
and it ought to be treated on chemical principles. An idea of the
composition of some of the more frequently occurring rocks may be
gathered from the following table, which represents rough averages:--

-----------------------------------------------------------------
| | |Oxide|Lime and |
|Silica.|Alumina.| of |Magnesia.|Alkalies.
| | |iron | |
-----------------------------------------------------------------
| % | % | % | % | %
Sandstone, grit, | | | | |
quartzite, &c. |80-100 | -- | -- | -- | --
Granite, gneiss, | | | | |
quartz-porphyry, | | | | |
fire-clay, &c. | 70-75 | 13-20 | 2 | 2 | 5-8
| | | | |Less in
| | | | |fire-clay.
Mica-schist | 65 | 18 | 5 | 3 | 3
Trachyte, syenite | 60 | 17 | 7 | 4-7 | 6-9
Clay-slate | 60 | 18 | 10 | 8 | 3