procedure for extracting the
gold held in combination with base metals is--first, reduction of the
particles to a uniform gauge and careful concentration only; next,
the dissipation, usually by simple calcination, of substances in the
concentrates inimical to the thorough absorption of the gold by the
mercury; and lastly, the amalgamation of the gold and mercury.

For general purposes, where the gangue has not been crushed too fine,
I think the Duncan pan will usually be found effective in saving the
concentrates. In theory it is an enlargement of the alluvial miner's tin
dish, and the motion imparted to it is similar to the eccentric motion
of that simple separator.

The calcining may be effectively carried out in an ordinary
reverberatory furnace, the only skill required being to prevent over
roasting and so slagging the concentrates; or not sufficiently calcining
so as to remove all deleterious constituents; the subject, however, is
fully treated in Chapter VIII.

For amalgamating I prefer some form of settler to any further grinding
appliance, but I note also improvements in the rotary amalgamating
barrel, which, though slow, is, under favourable conditions, an
effective amalgamator. The introduction of steam under pressure into an
iron cylinder containing a charge of concentrates with mercury is said
to have produced good results, and I am quite prepared to believe such
would be the case, as we have long known that the application of steam
to ores in course of amalgamation facilitates the process considerably.

Some seventeen years since I was engaged on the construction of a dry
amalgamator in which sublimated mercury was passed from a retort through
the descending gangue in a vertical cylinder, the material thence
falling through an aperture into a revolving settler, the object being
to save water on mines in dry country. The model, about quarter size,
was completed when my attention was called to an American invention,
in which the same result was stated to be attained more effectively by
blowing the mercury spray through the triturated material by means of
a steam jet. I had already encountered a difficulty, since found so
obstructive by experimentalists in the same direction, that is,
the getting of the mercury back into its liquid metallic form. This
difficulty I am now convinced can be largely obviated by my own device
of using a very weak solution of sulphuric acid (it can hardly be
too weak) and adding