The calculation of the results of a gravimetric assay has already been
referred to. If the result is to be stated as percentage, it may always
be done by the following rule:--_Multiply the weight of the substance
got by the percentage of metal it contains, and divide by the weight of
ore taken._

Gravimetric methods are divided into three groups: (1) mechanical
separations; (2) dry methods; and (3) wet methods.

~Mechanical Separations.~--Under this head are classed the method of
assaying tin ores, known as vanning, and the amalgamation assay for
gold. A set of sieves to determine the relative proportion of powders of
different degrees of fineness is sometimes useful. A set with 10, 20, 40
and 80 meshes to the inch is convenient.

~Dry Assays.~--An important distinction between wet and dry methods of
assaying is, that in the former the substance is got into the liquid
state by solution, whilst in the latter fusion is taken advantage of.

The difference between solution and fusion is easily illustrated: a lump
of sugar heated over a candle-flame melts or fuses; suspended in water
it dissolves. Many substances which are insoluble or infusible of
themselves, become soluble or fusible when mixed with certain others;
thus, in this way, solution is got with the aid of reagents, and fusion
with the help of fluxes. For example, lead is insoluble in water, but if
nitric acid be added, the metal rapidly disappears. It is convenient,
but somewhat inaccurate, to say that the acid dissolves the lead. If the
lead be acted on by nitric acid alone, without water, it is converted
into a white powder, which does not dissolve until water is added; in