Ivory is hard and elastic, and is considerably more tran sparent than white paper of equal thickness. The outside is covered by a cortical substance, softer than the rest ; brown or almost black externally, by which also the cavity is lined. In general, it is nearly a line thick, though some times scarcely perceptible on approaching the tip. The ivory, in both cases, is coarser ; that is, the grain is larger and more perceptible in the vicinity of the bark, softer, and, as we think, more readily discolours on exposure to the air. It progressively becomes finer on receding from the exter nal cortical part ; and where there is no sensible bark on the cavity, it seems to have attained the greatest perfection. Hardness and whiteness are the only properties desirable. But that is most prized which exhibits a kind of diaphanous appearance whin first cut asunder. The outside is always to be rejected, as also portions near the nerve.
The structure of elephants' ivory exhibits no important differences. The disk of a transverse section always pre sents a number of granulated lines in a zigzag direction, which gradually become more delicate on approaching the interior, until they totally disappear from the naked eye.
Cuvier affirms, that this character, common to all ele phants' ivory, and depending immediately on the pores of the pulpy nucleus of the tusk, is not to be found in the tusks of any other animal. We shall remark, in general, that the texture of the ivory of different animals which we have examined, is very different. On reducing a trans verse section of a portion of very fine ivory to the thickness of 100th part of an inch, we found innumerable concentric circles surrounding the cavity, disposed, as it were, in bands of irregular breadth. At this thickness, the ivory required a high polish to expose them ; and they were indistinct without the aid of a magnifier. But on reducing a piece greatly thinner, and through which common printing would easily be read when laid upon a page, they were readily seen with the naked eye. Very minute lozenges were ob served to be formed by intersecting curves, and many stria: or radiations, in which were minute square masses more opaque. Instead of the conical intersection, the grain near the bark exhibited waved white lines among the browner part. A longitudinal section of ivory presents somewhat of a lamellated aspect ; of uniform colour in a fine tusk, but sometimes more distinctly marked, and with yellowish edges in others. The texture resembles that of wood ; and, on examining a thin plate, it will be found to be extremely similar to the general appearance of ash, consisting of al ternate irregular darker and lighter streaks. By reducing it to extreme thinness, they become infinitely less percepti ble, and the ivory assumes a bluish transparency. It 'nay be thence understood, that the tusk is formed of innumera ble concentric circles, so intimately united as to constitute one solid and consistent whole. When in a state of decom
position by lying long in the earth, it separates into thin concentric conical plates ; and sometimes, though very rarely, we have observed a portion separate in this manner from recent ivory. Mr. Corse Scott is of opinion, that the age of an elephant might be deduced from the number of layers ; but it is not certain that they grow regularly year by year ; and besides, they appear different in a large and thick section than in one which is very thin. The nature of this annular formation is particularly' conspicuous in %s hat is called the tooth of the whale, which is probably a real tooth. The cortical part is very thick, and much whiter than the rest, and it is of about equal hardness. That of a section of an inch and a half in diameter we found to be an eighth of an inch thick, of a homogeneous surface to the naked rye, but seen to consist of the thinnest layers on the application of a magnifier. Instead of the granulated ap pearance and intersecting curves of elephants' ivory, the whole is disposed in numerous concentric rings, exactly resembling those which surround the pith of a tree, pro ceeding from the centre to the circumference. They are of unequal breadth, and seem arranged in grains, with an intermediate transparent ring. Very fine and delicate ra diations also proceed from the centre to the circumference, and a play of light is produced, which gives the surface somewhat the appearance of the cat's eye. Observations on plates of ivory are greatly facilitated by the application of oil or water, which produce such transparency as to ex pose the internal and also the superficial structure.
The teeth of the sea horse, morse, or trichecus rosmarus of Linnaeus, afford the hardest and whitest of all ivory. Some are reported to be two or three feet in length, and to weigh thirty pounds, though usually they do not exceed two or three. They are short, and very much curved. A large hoUow occupies the wide end, as in the tusk of the sea cow and the elephant ; the centre or nerve is of a dark triangu lar shade. A rough glassy enamel covers the cortical part, of excessive hardness, and which strikes lire with steel: It can scarcely be removed with a file, and blunts a tool of the highest temper. The texture of this ivory nearly resem bles that of the former in concentric layers; but they are as fine as human hairs, and, in a transverse section, are hardly perceptible by the naked eye. When brought into view by magnifiers, and a favourable light, they are exceedingly numerous, but of irregular diameter. Little is exposed by a longitudinal section, unless when very thin; and then appear what may be supposed the successive layers, pro ceeding diagonally from the core.