ove any coating of oxide.
When the mercury will absorb no more zinc, squeeze through chamois
leather or calico (as for silver amalgam), and well rub in. The plate
thus prepared should stand for a few days, dry, before using. If, before
amalgamation with gold takes place, oxide of copper or other scum should
rise on this plate a little very dilute sulphuric acid will instantly
remove it.

Sodium and cyanide of potassium are frequently used in dressing-plates,
but the former should be very sparingly employed, as it will often
do more harm than good by taking up all sorts of base metals with the
amalgam, and so presenting a surface which the gold will pass over
without adhering to. Where water is scarce, and is consequently used
over and over again, lime may be added to the pulp, or, if lime is not
procurable, wood ashes may be used. The effect is two-fold; the lime
not only tends to "sweeten" sulphide ores and keep the tables clean,
but also causes the water to cleanse itself more quickly of the slimes,
which will be more rapidly precipitated. When zinc amalgam is used,
alkalies would, of course, be detrimental.

When no other water than that from the mine is available, difficulties
often arise owing to the impurities it contains. These are various,
but among the most common are the soluble sulphates, and sometimes free
sulphuric acid evolved by the oxidisation of metallic sulphides. In the
presence of this difficulty, do one of two things; either _utilise_ or
_neutralise_. In certain cases, I recommend the former. Sometime since
I was treating, for gold extraction, material from a mine which was very
complex in character, and for which I coined the term "polysynthetic."
This contained about half a dozen different sulphides. The upper parts
of the lode being partially oxidised, free sulphuric acid (H2SO4) was
evolved. I therefore, following out a former discovery, added a little
metallic zinc to the mercury in the boxes and on the plates with
excellent results. When the free acid in the ore began to give out in
the lower levels I added minute quantities of sulphuric acid to the
water from time to time. I have since found, however, that with some
water, particularly West Australian, the reaction is so feeble (probably
owing to the lime and magnesia present) as to make this mode of
treatment unsuitable.

HOW TO MAKE A DOLLY

I have seen some rather elaborate dollies, intended to be worked with
amalgamating tables, but