The state in which the elements are present,
and the percentage (say of sulphides and sulphates) can in many cases be
determined; but this is not always required. When the difference between
the sum total and the elements found is small, it is reported as "oxygen
and loss." When, however, it is considerable, the oxygen may be reported
as such; and its amount be either determined directly in the way already
described, or calculated from the best determination that can be made of
the relative amounts of oxides, sulphides, sulphates, &c., present. Such
cases require a careful qualitative analysis to find out that the
substance is present; and then the separation of each constituent is
made as strictly as possible. These remarks apply especially to ores of
the heavy metals. The separation of the constituents is effected with
suitable solvents applied in proper order. The soluble sulphates, for
example, are extracted with water; the oxides by the dilute acids or
alkalies in which they are known to be soluble. The oxygen in the
sulphates and oxides thus obtained is estimated by determining the
sulphur and metals in the solutions, and calculating the amount of
oxygen with which they combine. The metals of the earths and alkalies
are almost invariably present as oxides, and are reported as such;
except it is known that they are present in some other form, such as
fluoride or chloride.
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