45 
are generally a combination of the cube with the octahe¬ 
dron, and may be split in the direction of the planes of 
the cube. It is the hardest and heaviest of all metals 
occurring in nature, and has a hardness of 6—7, a specific 
gravity of 21*101—24*95; it is neither soluble by nitric 
acid nor by nitro-hydrochloric acid, so that in a platinum 
solution it remains with the osm-iridium, from which it 
may be distinguished, in addition to its greater heaviness, 
by the form of the grains, since the latter ore always occurs 
in hexagonal tables and bluish-white laminae, with dis¬ 
tinctly foliated fracture. When melted with saltpetre it 
is partially dissolved in hot hydrochloric acid, forming with 
it a blue fluid. Before the blow-pipe it evolves no odour 
of osmium like the following mineral. It has, until the 
present time, only been found at Nischni-Tagilsk, and at 
Ava in Lower India; the Ural variety contains 76*80 of 
iridium, 19*64 of platinum, 0*89 of palladium, and 1*78 of 
copper. 
Osm-iridium is found in hexagonal laminae and in thin 
leaves of a silver-white colour passing into bluish (Plate 
XIII. Fig. 14), and may be recognised by the foliated 
structure ; the specific gravity is = 19*47—21*2, the hard¬ 
ness 6*0—7. There is a purer and a darker variety ; the 
latter is distinguished by its more leaden-grey colour, and 
the greater amount of osmium it contains. Both of them, 
when held in the platinum forceps, give off the peculiar 
odour of the osmium vapours, which is especially marked 
in the darker variety. Both are insoluble in nitro-hydro¬ 
chloric acid. Pure iridium has been applied to different 
metals as an alloy, especially to the improvement of steel, 
which is thereby made less liable to rust. 
Palladium. 
This precious metal is almost silver-white, and occurs 
in cubes, octahedrons, and small roundish grains of fibrous 
structure, as in Brazil, where it is found in the sand with 
platinum. It is also found in hexagonal laminae, im¬ 
bedded in the seleniurets of lead at Tilkerode in the Hartz; 
it is, therefore, dimorphous. The Brazilian variety always 
contains a small proportion of iridium and platinum. Its 
lustre is metallic, it is extremely malleable, of 4*5—5 hard¬ 
ness, and 11*5—11*8 specific gravity. The colour is some¬ 
what lighter than that of platinum. In nitric acid with 
difficulty, and in nitro-muriatic acid easily soluble, to a 
reddish-brown fluid. When mixed with a solution of 
iodine it becomes black, and when held in the alcoholic 
flame it forms a blackish carburet of palladium. Before 
the blow-pipe it is infusible. 
Palladium is generally obtained from rough platinum, 
and usually comes into commerce alloyed with a small 
proportion of gold. It has been applied to the manufac¬ 
ture of nautical instruments, and, when alloyed with gold 
and silver, for sockets for the pivots of watch wheels. 
Silver. 
A very widely distributed noble metal, sometimes 
occurring in large masses, which, by its pure white colour, 
malleability, sound, and high capability of polish, has been 
highly valued in all ages. It has, nevertheless, the pecu¬ 
liarity of becoming gradually tarnished or dull in the 
atmosphere, since it is decomposed by sulphuretted hydro¬ 
gen, which is never absent where animal life exists. It 
is distinguished from the other noble metals principally 
by the manner of its occurrence; for it is found, not only 
native and alloyed, in combination with chlorine, bromine, 
iodine, etc., but also frequently mineralised by sulphur. It 
has no hydrogen compounds. Its combinations with the 
electro-negative metals, as antimony, arsenic, and tellurium, 
are especially remarkable, and they not unfrequently occur 
like sulphur compounds with a metallic lustre and friability. 
Most of the silver ores occur in the primary and the older 
stratified rocks, principally in veins; and large quantities 
of silver are obtained from lead-glance, in which it exists 
as a sulphuret of silver. It does not occur in alluvial deposits. 
PLATE XIV. 
Fig. 1 — 5 .— Native Silver. 
One of the most valuable varieties, for it sometimes 
appears in considerable masses. It occurs crystallised in 
cubes (Figs. 2 and 3; Fig. 3 represents the foliated struc¬ 
ture in the direction of the cubic planes), in octahedrons, 
dodecahedrons, etc., stem-like, as in Fig. 1; in crystalline, 
often-streaked plates, as Fig. 4; arborescent, as Fig. 5; 
wire-like and capillary, and also in thick unshapely masses, 
especially associated with cobalt and lead ores, and some¬ 
times with the ores of copper and nickel; it is either 
silver-white, as in Figs. 1, 2, and 3, or with a reddish 
tarnish, as in Fig. 4; or with a blackish tarnish, as in 
Fig. 5; sometimes entirely coated with sulphuret of silver 
and blackened, as, for instance, in the rocks of Saxony. 
The fracture is hacked, the metallic lustre, especially in 
the streak, is a remarkable silver-white ; very malleable, 
hardness 2*5—3*0, specific gravity 10*3—10*8. Native 
silver consists either of pure silver, or it contains traces 
of gold, copper, arsenic, antimony, and sometimes even of 
platinum. Before the blow-pipe it is fusible without being 
changed. In pure nitric acid it is easily dissolved, the 
gold being left as a residue; the solution blackens the 
skin, and the silver is precipitated by copper as a metal; 
a solution of common salt, and free hydrochloric acid, give 
a white cheesy precipitate of chloride of silver when added 
to this solution. 
The occurrence of native silver is principally confined 
to veins in the igneous and transition rocks. At one 
time a large quantity of silver was found in the Black 
Forest, in the Sophia mines at Wittichen, at Wenzel near 
Wolfach, and St. Antoine; also in the Beinerzau. The 
delicate little tree (Fig. 5), which consists of small octa¬ 
hedrons, and lies on reddish heavy spar, is from Hein- 
richsgang at Wolfach. It is found in great plenty on 
the Erzgebirge, at Freiberg and Schneeberg, and at 
Kongsberg in Norway, from whence the specimens Fig. 
1—4 come, and where, in the year 1834, a mass of 
720 pounds was found. In the year 1477 a block of 
40,000 pounds weight was taken from the Markus mine 
