The hard solid substances derived from the Animal Kingdom, are parts of the external or internal skeletons, as shells and bones; or of the instruments of sustenance and defence, as horns, hoofs, nails, claws, and teeth: these, together with the various coverings of animals, whether hair, feathers or scales, are alike composed of animal and earthy matters, almost exclusively albumen, gelatine, and lime, combined in various proportions, and with a structure more or less interspersed with animal fibre. Many of these arc either formed by the deposition of successive annual layers, or they are altogether yearly renewed.

A brief consideration of the chemical difference between their component parts, and of their respective proportions, in such as are used in the arts, will show the reasons for their various characters, and different treatment with tools.

Albumen, the principal ingredient of these animal substances and which exists in the purest form in the white of eggs, is hardened by a degree of heat less than the boiling temperature of water, and is insoluble in the same. Gelatine, of which jelly and glue are different examples, is softened by heat, and rendered fluid by the addition of water; both are easily cut and scraped, in all their various stages from soft to hard, and during this change they contract very materially, but without entirely losing their elasticity.

* The author begs to premise, that having but little personal experience in the subject-matters of this present chapter, he has not hesitated to draw largely from the Manuel du Tourneur and also from two valuable papers " On Horn and Tortoiseshell," and " On Bone, and its uses in the Arts," by Arthur Aikin, Esq., read before the Society of Arts in 1832, 1838, and 1839, and published in their Transactions, Vol. LII., Part 2, pp. 334 - 379: in which, in addition to the information on the points here to bo discussed, are contained many interesting particulars, on the physiology, and on the historical and present uses of these substances. An article on Tortoiseshell, in Gill's Technological Repository, 1827, Vol. L, p. 332. (derived from the Franklin Journal and l' Encyclopedie Methodique,) has likewise been consulted: the several extracts will be respectively noticed.

The author has been further enabled by the kindness of various practical friends, to advance other examples and particulars, and to procure specimens of several of the materials in their different stages of manufacture. In accordance with the prescribed plan of the work, he has dwelt more at length on those parts which the amateur may practise with comparatively few apparatus.

The earthy matters of the animal solids, principally the phosphate and carbonate of lime, are widely different from the foregoing, and also from the substances of the woods and metals. They are inelastic, and often crystalline, and therefore incapable of being cut into shreds or shavings: as when they are divided, they become smaller fragments or particles which are always angular: they are comparatively uninfluenced by water or small changes of temperature, and are incapable of contraction.

When the earthy and crystalline structures prevail, the animal substances are harsh, incapable of absorbing moisture, or of alteration of size or form; when the animal and fibrous characters prevail, they are easily cut, and they absorb moisture, soften, and swell.*

In some of the shells, the quantity of animal matter is so small and the lime is in so hard and compact a form, that they are very brittle, partially translucent, generally they have smooth surfaces, and are incapable of being cut with a knife or tools; such shells are called porcelanous, from their resemblance to porcelain; they include most of the univalve shells, such as the whelks, limpets, and cowries. Most of these can only be worked upon after the manner of the lapidary, with emery and other gritty matters harder than themselves, by which means they are usual course in preparing the rough pearl shell for the arts, is to cut out the square and angular pieces with the ordi-nary brass-back saw, and the circular pieces, such as those for buttons, with the annular or crown saw, fixed upon a lathe mandrel. The sides of the pieces then ground flat upon a wet grindstone, the edge of which is turned with several grooves, as the ridges are cosidered to cut more quickly than the entire surface, from becoming less clogged with the particles ground off. The pieces are finished upon the flat side of the stone, and are then ready for Inlaying, engraving and polishing, according to the purposes for which they arc intended. Cylindrical pieces are cut out of the thick part of the shell, near the joint or hinge, and rounded upon the grindstone, ready for the lathe, in which they may be turned with the ordinary tools used for ivory and the hard-woods.

* The numbers attached to the following substances show, in a rough manner, the rate per cent, of animal matters respectively contained; the remainder, principally carbonate and phosphate of lime, being neglected.

Enamel of teeth, the hardest of the class, contains from 2 to 3 1/2 per cent, of animal matter, (Berzelius). Porcelanous shells are nearly similar. Nacreous shells, 24 per cent. (Hatchett). Ivory, 24 per cent. (Ure); 25 per cent. (Merat Guillot). Bone, 33 per cent. (Berzelius). Horn, is coagulated albumen and lime, with 1/2 per cent, of phosphate of lime, (Ure). Tortoiseshell is nearly the same as horn. The horn of the buck and hart are intermediate between bone and ordinary horn (Ure) cut and polished, as will be explained in speaking of that art; by analysis, porcelauous shells arc considered closely to agree with the enamel of the teeth.

The nacreous shells, thus named from nacre, the French for mother-of-pearl, are most commonly known in the shells of the pearl-bearing oyster of the Indian Seas (Ostraea margaritifera), but they include the generality of the bivalve shells, as the various oysters, muscles, etc.; within they are smooth and iridescent; without they have a rough coat or epidermis.

These kinds contain a larger proportion of animal matter, which is considered to be arranged in alternate layers with the carbonate of lime; and as these shells also arc impenetrate to water, they neither shrink nor swell. The pearl shells are less frangible and hard than the porcelanous shells, and they admit of being sawn, scraped, and filed, with ordinary tools; but they are harsh, scratchy, and disagreeable under the operation.

The beautiful iridescent appearance of the pearl shells is attributed to their laminated structure, which disposes their surfaces in minute furrows, that decompose and reflect the light; and owing to this lamellar structure, they also admit of being split into leaves, for the handles of knives, counters and the purposes of inlaying. As the pieces are very apt to follow, and even to exceed the curvature of the surface, splitting is not much resorted to, but the different parts of the shell are selected to suit the several purposes as nearly as possible; and the excess of thickness is removed upon the grindstone in preference to risking the loss of both parts in the attempt to split them.

In the bones of animals, the earthy and animal matters are more nearly balanced; they are therefore less brittle than the shells, but prior to being used they require the oil with which they are largely impregnated to be extracted by boiling them in water, and bleaching them in the sun or otherwise. This process of boiling, in place of softening, robs them of part of their gelatine, and therefore of part of their elasticity and contracti-bility likewise; they become more brittle, and having a fibrous structure, they break in splinters.

The forms of the bones are altogether unfavourable to their extensive or ornamental employment; most of them are very thin and curved, contain large cellular cavities for marrow, and are interspersed with vessels that are visible after they are worked up into brushes, spoons, and articles of common turnery. The buttock and shin bones of the ox and calf, are almost the only kinds used. To whiten the finished works, they are soaked in turpentine for a day, boiled in water for about an hour, and then polished with whiting and water.

* The following are considered by an experienced dealer to be the respective qualities of the pearl shells. The Chinese, from Manilla, are the best; they are fine, large, and very brilliant, with yellow edges. Singapore, fine large shells, dead-white. Bombay, a common article. Valparaiso, also common, with jet black edges. South Sea pearl shells, common, with white edges. - See note on page 42.

The very beautiful dark green pearl shells, are known as ear-shells or sea-cars; they are unlike the others in form, being more concave, and with small holes around the margin, and are the coverings of the Haliotis, found in the Californian, South African, and East Indian Seas. Cameos are cut in the conch-shell, Strombus Gigas, of the southern coast of America, and the West Indian Islands.

The Rev. Essex H. Bond informs me that he has seen the Chinese work the largest of known shells, the Chama Gigas of Linnaeus, the Tridacna Gigas of Lamarck, into snuff-bottles, tops of walking-sticks, bangles (a kind of bracelet,) and similar articles, some of which he possesses. The shell is a bivalve and not nacreous, generally white, sometimes pale blue; it may be beautifully polished, and is less readily scratched than mother-of-pearl; its localities are the Indian Seas, New Holland and the Red Sea, but the largest are obtained from Sumatra, one pair from whence, described in Sir Joseph Banks' MSS. Library, is said to weigh, the one valve 285, the other 222 pounds, but the more usual weight is about 100 pounds each valve. Mr. Bond considers the useful portions of the shell, already prepared, might be obtained from China.

Bone is far less disagreeable under the tools than the pearl shell, but it is nevertheless hard, harsh and chalky; the screws cut on bone are imperfect and soon injured. It is harder, often whiter, but much less pleasant to work than ivory, which beautiful material will be treated of separately in the next chapter.