ordinary temperature), are mainly used; and dry assays, those in which
solid re-agents are almost exclusively employed. Dry assays form a
branch of gravimetric work, and we shall include under this head all
those assays requiring the help of a wind furnace. Wet assays, as
generally understood, would include not only those which we class as wet
gravimetric assays, but also all the volumetric processes.

~Gravimetric Methods~ aim at the separation of the substance from the
other matters present in the ore, so that it may be weighed; and,
therefore, they must yield the _whole_ of the substance in a pure state.
It is not necessary that a metal should be weighed as metal; it may be
weighed in the form of a compound of definite and well known
composition. For example, one part by weight of silver chloride contains
(and, if pure, always contains) 0.7527 part of silver; and a quantity of
this metal can be as exactly determined by weighing it as chloride as by
weighing it in the metallic state. But in either case the metal or its
chloride must be pure.

Exact purity and complete separation are not easily obtained; and
methods are used which are defective in one or both of these respects.
It is well to note that an impure product increases the result, whilst a
loss of the substance decreases it; so that if both defects exist in a
process they tend to neutralise each other. Of dry methods generally, it
may be said that they neither give the whole of the substance nor give
it pure; so that they are only calculated to show the amount of metal
that can be extracted on a manufacturing scale, and not the actual
quantity of it present. Their determinations are generally rough and
always low. The gold and silver determinations, however, will compare
very favourably with any of the other processes for the estimation of
these metals in their ores.