IRl 
any heat Mr. Tennant could employ. It 
did not combine with sulphur, or with ar- 
senic. Lead unites with it easily, but is 
separated by cupellation, leaving the iri- 
dium on the cupel as a coarse black powder. 
Copper forms with it a very malleable al- 
loy, which, after cupellation with the addi- 
tion ot lead, leaves a small proportion of the 
iridium, but much less than in the preced- 
ing instance. Silver forms with it a per- 
fectly malleable compound, the surface of 
which is tarnished merely by cupellation ; 
yet tiie iridium appears to be diffused 
through it in fine powder only. Gold re- 
mains malleable, and little altered in co- 
lour, though alloyed with a considerable 
proportion ; nor is it separable either by 
'cupellation or quartation. If the gold or 
silver be dissolved, the iridium is left as a 
black powder. 
The French chemists observed, that this 
new metal gave a red colour to the triple 
salt of platina and sal ammoniac, was not 
altered by muriate of tin, and was precipi- 
tated of a dark brown by caustic alkali. 
Vauquelin added, that it was precipitated 
by galls, and by prussiate of potash : but 
Mr. Tennant ascribes this to some im- 
purity. 
Mr. Tennant gave it the name of iridium 
from the striking variety of colours it af- 
fords while dissolving in muriatic acid. 
Dr. Wollaston has observed, that among 
the grains of crude platina there are some 
scarcely distinguishable tfom the rest but 
by their insolubility in nitro-muriatic acid. 
They are harder, however, when tried by 
the file; not in the least malleable; and of 
the specific gravity of 19.5. These ap- 
peared to him to be an ore, consisting en- 
tirely of the two new metals. 
IRIS, in anatomy, the anterior coloured 
part of the uvea of the eye, so called be- 
cause of its variety of colours, iris being 
the Latin word for rainbow. The iris is a 
circular variously coloured part, which sur- 
rounds the pupil ; it is in some persons blue, 
in others black, brown, grey, &c. each of 
which has its peculiar beauty, and is suited 
to the complexion of the person who has it. 
See Anatomy, Optics. 
IRIS, in botany, a genus of the Trian- 
dria Monogynia class and order. Natural 
order of Ensatas. Irides, Jussieu. Essen- 
tial character : corolla six-peta!led, une- 
qual, petals alternate, jointed and spread- 
ing; stigmas petal-form, cowled, two-lipped. 
There are fifty species. The iris is an ii ■ha- 
bitant of every quarter of the world ; Arr.e- 
IRO 
rica, however, produces very few. Several 
are found natives of the colder regions of 
Asia, more still of Europe, and most of the 
Cape of Good Hope. These plants are 
herbaceous flowering perennials, both of 
the fibrous, tuberous, and bulbous rooted 
kind, producing thick annual stalks from 
three inches to three feet in height, termi- 
nated by large liexapetalous flowers, having 
three of the petals reflexed back and three 
erect: these are very ornamental plants, 
appearing in flower in May, June, and July. 
IRON is a metal of a bluish white co- 
lour, of considerable hardness and elasticity ; 
very malleable, exceedingly tenacious and 
ductile, and of a moderate specific gravity 
among metallic substances. It is much 
disposed to rust by the access of air, or the 
action of water, in the common tempera- 
ture of the atmosphere. The appearance 
of prismatic colours on its polished surface 
takes place long before ignition ; and at so 
low a temperature, that the slightest coat- 
ing of grease is sufficient to prevent their 
appearance by defending it from the con- 
tact of air. It may be ignited, or at least 
rendered sufficiently hot to set fire to brim- 
stone, by a quick succession of blows with 
a hammer. When struck with a flint, or 
other hard stone, it emits decrepitating 
ignited particles, such as can be obtained 
from no other metal by the same means. 
These particles are seldom larger than the 
two hundredth part of an inch in diameter ; 
and, when examined by a magnifier, are 
found to be hollow, brittle, and of p. grey- 
ish colour, resembling the scales of burned 
iron. This metal is easily oxided by fire. 
A piece of iron wire, immersed in ajar of 
oxygen gas, being ignited at one end, will 
be entirely consumed by the successive 
combustion of its parts. It requires a very 
intense beat to fuse it ; on which account it 
can only be brought into the shape of tools 
and utensils by hammering. Tins high de- 
gree of infusibility would deprive it of the 
most valuable property of metals, namely, 
the uniting of smaller masses into one, if it 
did not possess another singular and advan- 
tageous property, which is found in no 
other metal except platina ; namely, that of 
welding. In a white heat, iron appears as 
if covered with a kind of varnish ; and in 
this state, if two pieces be applied together, 
they will adhere, and may be perfectly 
united by forging. Iron is thought to be 
the only substance in nature, which has the 
property of becoming magnetical. It is 
highly probable, from the great abundance 
