Some
perliminary observations on the spectrum of the vapour at the
temperature of the oxyhydrogen flame, made by Lockyer and Roberts, (a) showed that there was a distinct absorption both at the blue and at the red end.
The solvents for gold are given in the article Chemistry, vol. v. p. 529. It
may be added that finely-divided gold dissolves when heated with strong
sulphuric acid and a little nitric acid. Dilution with water, however,
precipitates the metal as a violet or brown powder from the solution
so obtained. Gold is also attacked when strong sulphuric acid is
submitted to electrolysis with a gold positive pole. (b) \V. Skey has shown (c) that
in substances which contain small quantities of gold, the precious
metal may be removed by the solvent action of a tincture of iodine or
bromine in water. Filter paper soaked with the clear solution is burnt,
and the presence of gold is indicated by the colour of the ash.
Occlusion of Gas by Gold.—Graham has shown (a) that
gold is capable of occluding 0'48 of its volume of hydrogen, and 0 -0
of its volume of nitrogen. Varrentrapp has also pointed out that "
cornets" from the assay of gold may retain gas if they are not strongly
heated. Artificial crystals of gold may be formed when the molten metal
is slowly cooled.
Occurrence and Distribution.—Gold
is found in nature chiefly in the metallic state, or as native gold,
and less frequently in , combination with tellurium, lead, and silver,
forming a peculiar group of minerals confined to a few localities in
Europe and America. These are the only certain examples of natural
combinations of the metal—the minute although economically valuable
quantity often found in pyrites and other sulphides being probably only
present in mechanical suspension, although for practical purposes it
may be spoken of as combined. The native metal occurs tolerably
frequently in crystals belonging to the cubic system, the octahedron
being the commonest form, but other and complex combinations have been
observed. Owing to the softness of the metal, large crystals are rarely
well defined, the points being commonly rounded. In the irregular
crystalline aggregates branching and moss-like forms are most common,
and in Transylvania thin plates or sheets with diagonal structures are
characteristic. These have recently been shown by Vom Rath to be
repeated combinations of distorted tetrahexahedra. During the
preparation of a mass of pure gold in the Mint at London, some fine
crystals which appear to be aggregations of octahedra were obtained;
and dendritic crystals of gold prepared artificially, have been
described by Chester. It is possible also to obtain gold in crystals by
heating its amalgam; according to Knaffl, an amalgam of 1 part of gold
with 20 parts of mercury is maintained at a temperature of 80° C for
eight days. It is then heated to 8o° C. with nitric acid of specific
gravity 1*35, when dull crystals will be left, which become brilliant
when more strongly heated. More characteristic, however, than the
ciystallized are the irregular forms, which, when large, are known as
"nuggets" or "pepites," and when in pieces below \ to h ounce
weight as gold dust, the larger sires being distinguished as coarse or
nuggety gold, and the smaller as gold dust proper. Except the larger
nuggets, which may be more or less angular, or at times even masses of
crystals, with or without associated quartz or other rock, gold is
generally found bean-shaped or in some other flattened form, the
smallest particles being scales of scarcely appreciable thickness,
which, from their small bulk as compared with their surface, subside
very slowly when suspended in water, and are therefore readily carried
away by a rapid current. These form the "float gold" of the miner. The
physical properties of native gold are gener-
