of
gold leaves may be still further reduced by floating them on a dilute
solution of cyanide of potassium. When very thin, leaf gold appears
yellow by reflected and green by transmitted light. If, however,
certain gold films are heated, the light transmitted is ruby red; the
pressure of a hard substance on the film so changes its state of
aggregation that green light is again transmitted, (a) The metal is extremely ductile; a
single grain may be drawn into a wire 500 feet in length, and an ounce
of gold covering a silver wire is capable of being extended more than
1,300 miles. Gold can readily be welded cold, and thus the
finely.divided metal in the state in which it is precipitated from
solution may be compressed between dies into discs or medals. According
to G. Ross, {/>) the specific gravity of gold in the
finely-divided state in which it is precipitated from solution by
oxalic acid is 19'49. The specific gravity of cast gold varies from
18'29 to 1937, and by compression (c) between dies the specific
gravity may be raised from 19'37 to 19'41; by annealing, however, the
previous density is to some extent recovered, as it then is found to be
19'10. Its atomic weight is variously given as follows :—19667 (Berzelius), 196-3 (Levol), 190 5 (Wurtz), 196-0 (Watts). The number adopted in this work (Chemistry, vol. v., p. 528) is 196-2. Different observers have given the following temperatures as its melting point:—1,425° (C. Daniell), 1,200° C. (Pouillet), 1,380° C. (Guyton de Morveau). Rimesdijk, (d) after comparing the several results, concludes that it may be considered to be 1,240° C. The electric conductivity is
given by Matthiessen as 73-99 at 15-1° C, pure silver being 100; this
depends greatly on its degree of purity,—the presence of a few
thousandths of silver lowering its conductivity by ten pM cent. The specific resistance of the metal in electromagnetic measure, accordrSg to the cenlimetre-gramme-second system of units, is 2,154. Its conductivity for heat is 53-2 (Wiedemann and Franz), pure silver being 100. Its specific heal is 0-324 (Regnault). Its co-efficient of expansion for
each degree between 0° and 100" C. is 0000014661, or for gold which has
been annealed 0-000015136 (Laplace and Lavoisier). The specific magnetism of the metal is 3-47 (Becquerel). Details as to its tenacity and rigidity are given in the article Elasticity. With regard to its volatility, Gasto Claveus (e) states
that he placed an ounce of pure gold in an earthen vessel in that part
of a glass-house where the glass is kept constantly melted, and
retained it in a state of fusion for two months without the loss of the
smallest portion of its weight. Kunkel describes a similar experiment,
which was attended with the same result. Homberg, (/) however, observed
that when a small portion of gold is kept at a violent heat, part of it
is volatilized. Both Macquer and Lavoisier showed that when gold is
strongly heated, fumes arise which gild a piece of silver held in them.
Its volatility has also been studied by Elsher, and, in the presence of
other metals by Napier, (g) Hellot affirms that when an alloy
of 7 parts of zinc and 1 part of gold is heated in air, the whole of
the gold rises in the fumes of oxide of zinc which are produced. Gold
is dissipated by sending a powerful charge of electricity through it
when in the form of leaf or thin wire. In the gold spectrum Muggins
has observed twenty-three lines, and the wave lengths of the three most
important of these are 5,231, 5,835, and 6,276 respectively.
