mo 



pound, the surface of which is tarnished 

 merely by cupellation : yet the iridium 

 appears to be diffused through it in fine 

 powder only. Gold remains malleable, 

 and little altered in colour, though alloy- 

 ed with a considerable proportion ; nor 

 is it separable either by cupellation or 

 quartation. If the gold or silverbe dissolv- 

 ed, 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 

 precipitated of a dark brown by caustic 

 alkali. Vauquelin added, that it was pre- 

 cipitated by galls, and by prussiate of pot. 

 ash : but Mr. Tennant ascribes this to 

 some impurity. 



Mr. Tennant gave it the name of iri- 

 dium, from the striking variety ot'colours 

 it affords while dissolving in muriatic acid. 



Dr. Wollaston has observed, that among 

 the grains of crude platina, there are some 

 scarcely distinguishable from 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 1^.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 

 surrounds 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 ANATOJIY, OPTICS. 



IRIS, in botany, a genus of the Trian- 

 dria Monogynia class and order. Natural 

 order of Ensatse. Irides, Jussieu. Essen- 

 tial character : corolla six-petalled, une- 

 qual, petals alternate, jointedand spread- 

 ing, stigmas petal-form, cowled, two-lip- 

 ped. There are fifty species. The iris 

 is an inhabitant of every quarter of the 

 world ; America, however, produces very 

 few. Several are found natives of the 

 colder regions of Asia, more still of Eu- 

 rope, 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, terminated by large hexa- 

 petalous flowers, having three of the pe- 

 tals 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 elasti- 



IRO 



city ; rery malleable, exceedingly 

 cious and ductile, and of a moderate spe- 

 cific gravity among metallic substances. 

 It is much disposed to rust by the acc--ss 

 of air, or the action of water, in the com- 

 mon temperature of the atmosphere. 

 The appearance of prismatic colours on 

 its polished surface takes place long be- 

 fore ignition ; and at so low a tempera- 

 ture, that the slightest coating of grease 

 is sufficient to prevent their appearance, 

 by defending it from the contact of air. 

 It may be ignited, or at least rendered 

 sufficiently hot to set fire to brimstone, 

 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 hundreth part of an inch in dia- 

 meter; and, when examined by a magni- 

 fier, are found to be hollow, brittle, and 

 of a greyish colour, resembling the scales 

 of burned iron. This metal is easily ox- 

 ided by fire. A piece of iron wire, im- 

 mersed in a jar of oxygen gas, being ig- 

 nited atone end, will be entirely consum- 

 ed by the successive combustion of its. 

 parts. It requires a very intense heat to 

 fuse it ; on which account, it can only be 

 brought into the shape of tools and uten- 

 sils by hammering. This high degree of 

 inftisibility 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 ad- 

 vantageous property, which is found in 

 no other metal, except platina; namely, 

 that of welding. In a white heat, iron ap- 

 pears 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 na- 

 ture, which has the property of becom- 

 ing magnetical. It is highly probable, from 

 the great abundance of this metal, that all 

 substances which exhibit magnetism do 

 contain iron ; but it must be confessed, 

 that there remain many experiments to 

 be made among the earths and powders 

 which exhibit magnetical properties, be- 

 fore this negative proposition, which con- 

 fines magnetism to iron, can be admitted 

 as proved. 



When iron is exposed to the action of 

 pure water, it acquires weight by gradual 

 oxydation, and hydrogen gas escapes: 

 thi's is a very slow operation. But if the 

 steam water be made to pass through 

 a red hot gun-barrel, or through an ig< 

 nited copper or glass tube, containing 



