FACTITIOUS IVORY.-EGGS OF BIRDS.

155

This cement is made of isinglass, (which is prepared from the sound or swimming-bladder of the sturgeon,) dissolved in diluted spirits of wine, or more usually in common gin. The two are mixed in a bottle loosely corked, and gently simmered in a vessel containing boiling water; in about an hour the singlass will be dissolved and ready for use; when cold, it should appear as an opaque, milk-white, hard jelly; it is remelted by immersion in warm water, but the cork should be at the time loosened; it may be found necessary, after a time, to add a little spirit to replace that lost by evaporation. When the isinglass is dissolved in water alone, it soon decomposes.

Factitious ivory, and also factitious tortoiseshell, have been prepared in France in thin plates or veneers. I have procured some pieces about a foot square, of the so-called ivory; it looks exactly like a dull opaque cement, the tortoiseshell like a piece of stained horn; neither of them at all approach the beauty of the true substances.

Having adverted in the present and last chapter to many animal substances suitable to the mechanical arts, obtained from various inhabitants of the land and water, let me in conclusion mention some that are obtained from the feathered tribes, namely the eggs of birds, which although of limited application in the arts of embellishment, have at all ages served as models or standards of beautiful form.

They may be made to answer in a very perfect manner for the bodies of vases, the feet and upper parts being turned out of wood or ivory; for this purpose the egg-shells have been commonly used in their entire state; a hole being made at the top and bottom for the extraction of their contents, and the attachment of the remaining parts. I have now the pleasure to bring before the reader a method of cutting the shells of the eggs of our various domestic fowls, and other birds, for the formation of vases with detached covers.

Mr. G. D. Kittoe, the inventor of this ingenious, skilful, and elegant application of the lathe, has at my request kindly furnished the original sketches of the figures and the annexed description.

"In the accompanying drawing is represented the nose of a lathe with an egg chucked ready for cutting. Fig. 56, is the sec

d

A

A

156

MODE OF DIVIDING EGG-SHELLS FOR

tion of a chuck for holding the eggs to prepare them for the chuck represented in fig. 57.

"Fig. 56, is what is generally termed a spring chuck, and is made by rolling stout paper with glue, upon a metal or hardwood cylinder, the surface of which has been greased to prevent the paper adhering to it, and upon which it must remain until perfectly dry; when it may be removed, and cut or turned in the lathe as occasion may require."

a

Figs. 57.

56.

B

"This sort of chuck is very light, easily made, and well adapted for the brittle material it is intended to hold. Before fixing the egg in it, the inner surface should be rubbed with some adhesive substance, (common diachylon answers exceedingly well;) when this is done, the egg should be carefully placed in the chuck, the lathe being slowly kept in motion by one hand, whilst with the other the operator must adjust its position, until he observes that it runs perfectly true; then with a sharp-pointed tool he must mark the center, and drill a hole sufficiently large for the wire in the chuck, (fig. 57,) to pass freely through."

"When this is done, the egg must be reversed, and the same operation repeated on the opposite end; its contents must then be removed by blowing carefully through it; it is now ready for cutting, for which purpose it must be fixed in the chuck shown in fig. 57, which is made as follows:"

66

A, is a chuck of box, or other hard wood, having a recess turned in it at a, b, into which is fitted a piece of cork, as a soft substance for the egg to rest against. B, is a small cup of wood, with a piece of cork fitted into it, serving the same purpose as that in A. A piece of brass, d, is to be firmly screwed into the chuck A, and into that a steel wire, screwed on the outer end; on this a small brass nut, c, is fitted to work freely in a recess

VASES WITH DETACHED COVERS.

157 in the piece B; when the egg is threaded on the wire through the holes which have been previously made in it, this nut is to be gradually tightened up until it presses the cup, B, against the egg, sufficiently to hold it steady and firm enough to resist the action of a finely-pointed graver used to cut it."

The tool requires to be held very lightly, as a little undue violence would crush the shell; neither should the latter be pinched unnecessarily tight in the chuck, as otherwise when the point of the tool divides the shell, the two parts might spring together and be destroyed by the pressure.

It requires some delicacy of hand to attach the rings to the edges of the shell to constitute the fitting; the foot and top ornament are fixed by very fine ivory screws, the heads of which are inserted within the shell *.

In the next chapter I have to proceed to the materials obtained from the Mineral Kingdom.

* Mr. Kittoe has done me the favour to give me a very elegant example of this fragile manufacture, one rather different from those to which his mind is professionally directed in his study as an engineer, in Messrs. Maudslay's Works. He informs me, the shells, when soiled with the fingers, are the most safely cleaned with a solution of citric acid, and that even the eggs of the sparrow may be successfully treated, as above.

CHAPTER VIII.

MATERIALS FROM THE MINERAL KINGDOM.

THE materials from the mineral kingdom may be divided so far as regards these pages into two groups; the earthy, and the metallic.

The earthy materials, the subject of this chapter, when employed in the mechanical and useful arts, are generally used in their natural states.

The metallic minerals, consist in general of metallic oxides, combined with a larger quantity of some base, such as silex, clay, sulphur, &c., which are the most common mineralizers; the cohesion of the mass, has in general to be overcome by heat, which destroys the affinity of the component parts, and allows of the separation of the metals, in various ways. Of these processes I shall have scarcely anything to say; but the metals themselves, when obtained, will be treated of at some length in the succeeding chapters.

The earthy and crystalline mineral substances are less frequently worked by the amateur, than the metals, woods, &c., and therefore they will be noticed rather briefly, and in the order of their hardness, as derived from the following table. The instructions on a variety of processes of grinding and polishing these earthy substances, will be found in the chapter exclusively devoted to that subject under its numerous modifications, as applied to the materials of the three kingdoms generally; both as the exclusive means for the production of form, and also for the embellishment of surface, or polishing.

THEIR TREATMENT DEPENDENT ON STRUCTURE.

159

The above table exhibits the relative degrees of hardness of the several substances in the estimation of the mineralogist; thus tale may be scratched by gypsum, gypsum may be scratched by calcareous spar, the last by fluor spar, and so on throughout; in the next column are given some of the metals, &c. of about the same degrees of hardness; and the last column contains the number of minerals, which in respect to hardness, are ranked under each of the ten grades.

In the several practices of working these numerous substances, structure must also be taken into account, or the mode in which their separate particles are combined; thus hardened-steel, quartz, granite, and sand-stone, are each included under the number 7. The particles of the steel, however, are much more firmly united than those of the glassy crystalline quartz, which is far more brittle; and still more so than the aggregations of crystals in the granites; the last may be wrought by sharp-pointed picks, and chisels of hard steel, which crush and detach, rather than cut the crystals; and although sandstone consists almost entirely of particles of silex, cemented with silex, still the grains of the sandstone being but loosely held together, it may be turned with considerable facility with the tools used for turning marble, and which is the every day practice in turning the grindstone. Whereas granite, which contains from half to three-fifths felspar, a substance softer than silex, and porphyry which consists of crystals of felspar embedded in a base of felspar, cannot be turned with steel tools at all.

Several mineral substances are formed by the successive deposition of their component parts in uniform layers, as in mica and slate; or in alternate depositions, as in the Yorkshire flags or sandstones. The mica may in consequence be split into leaves even so thin as the one 50,000th of an inch; it is used by the optician, &c., often under the name of talc; the slate may be split into very thin leaves of considerable size; and those sandstones which result from the decomposition of granite, are most readily split through the layers of minute scales of mica, which being a lighter substance than the other ingredients, are deposited in separate layers.

Many hard substances, as the agates, cornelians, flints, &c., show neither the crystalline nor lamellar structure, and break with a fracture termed the conchoidal, of which the broken