A U R 
A U R 
leagues of vaporised air raised from the ocean 
between the tropics, accounts for the aurora 
borealis ; and that it appears lirst where it is 
first in motion, namely, in the most northern 
part; and the appearance proceeds south- 
warti, though the fire really moves north- 
ward. Franklin’s Exper. and Obs. 1769, 
p. 4<J. Philos. Trans, vol. lviii. p. 358, 784; 
lb. vol. li. p. 403 ; Lettere dell’ Ellettricismo, 
p. 269 ; or Priestley’s Hist, of Electricity. 
Mr. Dalton, who has paid considerabie at- 
tention to meteorology, gives the following 
account of the appearances of the aurora bo- 
? realis : — They come under four different de- 
[ scriptions. i. A horizontal light, like the 
break of day. 2. Fine slender luminous 
| beams, .well defined, and of dense light, 
I which often continue nearly a minute at 
rest. 3. Flashes pointing upwards, or in the 
direction of the beams which they succeed. 
These are only momentary, and have no la- 
teral motion. They appear broad and dif- 
| fuse, and of a weaker light than the beams ; 
they grow gradually fainter till they disap- 
1 pear, and continue for hours flashing at inter- 
vals. 4. Arcs nearly in the form of a rain- 
bow: these when complete go quite across 
the heavens, from one point of the horizon to 
the opposite point. These appearances ge- 
I nerally succeed each other in the following 
order: 1, the taint rainbow-like arcs; 2, the 
beams; and, 3, the flashes. The northern 
horizontal light appears to consist of an abun- 
dance of flashes or beams blended together 
by the situation of the observer. The beams 
of the aurora appear at all places to be arcs 
j of great circles of the sphere, with the eye in 
the centre ; and these arcs, if prolonged up- 
I wards, would all meet in a point. The rain- 
bow-like arcs cross the magnetic meridian at 
right angles. When two or more appear at 
once, they are concentric, and tend to the 
-east and west : also the broad arc of the ho- 
rizontal light tends to the magnetic east and 
west, and is bisected by the magnetic meri- 
dian ; and when the aurora extends over any 
part of the hemisphere, the line separating 
the illuminated part of the hemisphere from 
the clear part is half the circumference of a 
great circle crossing the magnetic meridian 
at right angles, and terminating in the east 
and west. That point in the heavens to 
which the beams of the aurora appear to con- 
verge, at any place, is the same as that to 
which the south pole of the dipping needle 
points at that place. The beams appear to 
rise above each other in succession; so that 
of any two beams, that which has the higher 
base has also the higher summit. Every beam 
appears broadest at or near the base,, and to 
grow narrower as it ascends ; so that the con- 
tinuation of the bounding lines would meet 
in the common centre 'to which the beam 
tends. 1 he height of the rainbow-like arcs 
of the aurora borealis is estimated by Mr. 
Dalton to be 150 miles above the earth’s sur- 
face. 
AURU M, gold. Gold seems to have been 
known from the very beginning of the world. 
Its properties and its scarcity have rendered 
it more valuable than anv other metal. 
It is of an orange reel or reddish yellow 
colour, and has no perceptible taste or 'smell. 
Its lustre is considerable, yielding only to 
that ot platinum, steel, silver, and mercury. 
Its specific gravity 19.3. No other substance 
js equal to it in ductility and malleability. I. 
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may be beaten out into leaves so thin that 
one grain of gold will cover 5&§ square inches. 
1 hese leaves are only 1-28 2000th of an inch 
thick, but the gold leaf with which silver wire 
is covered lias only l-12th of that thickness. 
An ounce of gold, upon silver wire, is capable 
of being extended more than 1300 miles in 
length. Its tenacity is considerable, though 
in this respect it yields to iron, copper, pla- 
tinum, and silver. From the experiments of 
Sickingen it appears that a gold wire 0.078 
inch in diameter is capable of supporting a 
weight of 150.07 lbs. avoirdupois, without 
breaking. It melts at 32° of Wedgewood’s 
pyrometer. When melted, it assumes a 
bright bluish green colour. It expands in 
the act of fusion, and consequently contracts 
while becoming solid more than most metals; 
a circumstance which renders it less proper 
for casting into moulds. It requires a very 
violent heat to volatilize it ; it is, therefore, 
to use a chemical term, exceedingly fixed. 
Boyle and Kunkel kept it for some months in 
a glass-house furnace, and yet it underwent 
no change ; nor did it lose any perceptible 
weight, after being exposed for some hours 
to the utmost heat of Mr. Parker’s lens. 
Homberg, however, observed, that when a 
very small portion of gold is kept in fusion, 
part of it is volatilized.' This observation was 
confirmed by Macquer; who observed the 
metal rising in fumes to the height of five or 
six inches, and attaching itself to a plate of 
silver, which it gilded very accurately ; and 
Mr. Lavoisier observed the very same thing 
when a piece of silver was held over gold 
melted bv fire blown by oxygen gas, which 
produces a much greater heat than common 
air. After fusion, it is capable of assuming a 
crystalline form. 
Gold is not in the least altered by being 
kept exposed to the air; it does not even 
lose its lustre; neither has water the small- 
est action upon it. It is capable, however, 
of combining with oxygen, and even of un- 
dergoing combustion in particular circum- 
stances. 'Fiie resulting compound is an ox- 
ide of gold. There are two oxides of gold ; 
the protoxides is of a purple or violet, the 
peroxide of a yellow colour. 
Gold must be raised, to a very high temper- 
ature before it is capable of abstracting oxy- 
gen from common air. It may be kept red 
hot almost any length of time without any 
such change. Homberg, however, observed, 
that when placed in the focus of Tschirnhau- 
sen’s burning glass, its surface became co- 
vered with a purple-coloured oxide ; and the 
truth of his observations were put beyond 
doubt by the subsequent experiments of 
Macquer with a still more powerful burning 
glass. It was remarked also in 1773 by 
Camus, that when the electric explosion is 
transmitted through gold leaf placed between 
two plates of glass, or when a strong charge 
is made to fall on a gilded surface — in both 
cases the metal is oxidized, and assumes a 
purple colour. The reality of the oxidize- 
ment of gold by electricity was disputed by 
some philosophers, but it has been put be- 
yond the reach of doubt by the experiments 
of Van Mariun,t When that celebrated phi- 
losopher made electric sparks from the pow- 
erful Teylerian machine pass through a gold 
wire suspended in the air, it took fire, burnt 
with a green-coloured flame, and was com- 
pletely dissipated in fume, whfoh when col- 
A U R 1 8 i) 
leefed proved to be a purple-coloured oxide 
of gold. This combustion, according to Van 
Marum, succeeded not only in common air, 
but also when the wire was suspended in hy- 
drogen gas and other gases which are not ca- 
pable of supporting combustion. These sin- 
gular observations would require to be veri- 
fied by other experiments before any conclu* 
sion can be drawn from them: The combus- 
tion of gold is now easily effected by exposing 
gold-leaf to the action of the galvanic pile. 
It has been made to burn with great bril- 
liancy by exposing a gold wire to the action 
of a stream of oxygen- and hydrogen gas 
mixed together and burning. In all cases of 
the combustion of gold, it has been ascer- 
tained, that the protoxide or purple-coloured 
oxide only is formed. The peroxide, or yel- 
low-coloured oxide, may be procured in the 
following manner : Equal parts of nitric and 
muriatic acids are mixed together, and pour- 
ed upon gold ; an effervescence takes place, 
the gold is gradually dissolved, and the li- 
quid assumes a yellow colour. It is easy to 
see in what manner this solution is produced^ 
No metal is soluble in acids till it has been 
reduced to the state of an oxide. There is a 
strong affinity between the oxide of gold and 
muriatic acid. The nitric acid furnishes oxy- 
gen to the gold, and the muriatic dissolves the 
oxide as it forms. When nitric acid is deprived 
of the greater part of its oxygen, it assumes a. 
gaseous form, and is then called nitrous gas, 
or more properly nitric oxide gas. It is the 
emission of this gas which causes the effer- 
vescence. The oxide of gold may be preci- 
pitated from the nitro-muriatic acid, by pour- 
ing in a little potash dissolved in water, or,, 
which is much better, a little lime ; both of 
which have a stronger affinity for muriatic 
acid than the oxide has. 
The oxides of gold may be decomposed in 
close vessels by the application of heat. The 
gold remains fixed, and the oxygen assumes 
the gaseous form. They may be decom- 
poser!, too, by all the substances which have 
a stronger affinity for oxygen than gold has. 
Hitherto gold has not been found capable 
of combining with sulphur, carbon, or hy- 
drogen. M. Pelletier combined it with phos- 
phorus, by melting together in a crucible 
half an ounce of gold and an ounce of phos- 
phoric glass, surrounded with charcoal. The 
phosphuret of gold thus produced was brittle,, 
whiter than gold, and had a crystallized ap- 
pearance. It was composed of 23 parts of 
gold and one of phosphorus. He formed 
the same compound by dropping small pieces 
of phosphorus into gold in fusion. Phos- 
phorus, then, is the only one of the simple 
combustibles with which gold at present is 
supposed capable of combining. 
Neither does gold combine, as far as is 
known, with either of the simple incombus- 
tible bodies. But gold combines readily with 
the greater number of the metals, and' forms 
a variety of alloys. 
The affinities of gold and its oxides are 
placed by Bergman in the following order:- 
Gold. 
Oxide of Gold. 
Mercury, 
Muriatic acid, , 
Copper, 
Nitric, 
Silver, 
Sulphuric, 
Lead, 
Arsenic, 
Bismuth. 
Fluoric, 
Tin, 
Tartaric, 
