a dental substance, placed by chemists between bone and horn; that variety which is distinguished by the decussating curved lines on the surface of the transverse section of the tusk is peculiar to the African and Asiatic elephants; but the tusks of the walrus, the narwhal, and the hippopotamus, and the teeth of the same animals, also furnish ivory. The fruit of a species of palm (Phytelephas macrocarpa) from Peru, is imported under the name of vegetable ivory. The tusks of the elephant furnish the chief supply of ivory. In the Great Exhibition of 1851 were a pair weighing 325 lbs., obtained from an animal killed near the Lake Ngami, in South Africa; each tusk measured 8 feet 6 inches in length, and 22 inches in basal circumference. It is considered that the tusks deteriorate in length and quality by the domestication of the elephant. African tusks are preferred to the Indian as being denser in texture, and not so liable to turn yellow. The demand for ivory for knife-handles, billiard-balls, keys of musical instruments, mathematical scales, chess-men, for inlaying, for thin plates for miniatures, for carving, and for various other purposes is so enormous, and the supply is so well maintained, that surprise may well be felt that the noble race of animals which yield it has not long since become extinct, especially when we consider that the supply of small tusks annually imported is so vast as to reduce the average weight of the whole import to about 9 lbs. per tusk. Teeth under from 10 to 16 lbs. are called scirclolosses. A few years ago it was calculated that the consumption of ivory in Sheffield alone required the annual slaughter of 22,000 elephants. The collection of broken or shed tusks may somewhat mitigate the zeal of the hunter, and we may also refer to that remarkable source of ivory which exists in Eastern Siberia and the Arctic marshes, where the tusks of the mammoth, and other animals, are collected in vast quantities, and sold under the name of Siberian Fossil Ivory, although they do not appear to have undergone that species of transmutation implied by the term fossil, but are as useful to the artist or manufacturer as recent ivory. We are not aware that fossil ivory is imported into this country, although it is extensively used by the ivory-workers on the continent.

The engine-turned markings on true ivory render it unfit for the purposes of the dentist, who accordingly prefers the teeth of the hippopotamus or river-horse, especially the two external semicircular teeth in the lower jaw. This ivory is known by the inaccurate term of sea-horse; it is covered with a thick enamel, which is so hard as to resist steel tools, and even to strike fire with steel, and is removed by means of a grindstone, or by driving a thin chisel between it and the ivory within. The action of the blow-pipe is also used for the same purpose. This kind of ivory is harder than that of the elephant, whiter in colour, and of more than double the value; it has a slight blue tint, and scarcely any grain. The dentist also makes use of the teeth of the walrus or sea-cow. In this case the outer portion consists of ivory, while the brown oval centre resembles bone. Small supplies of ivory are obtained from the long, straight tusks of the narwhal or sea-unicorn, the masticating teeth of the spermaceti whale, and the grinders of the elephant.

Regarding the elephant's tusk as ivory, properly so called, we may distinguish between African and Asiatic ivory by the former being, when recently cut, of a mellow, warm, transparent tint, with scarcely any appearance of grain, in which state it is called transparent or green ivory; but, as the oil dries up by exposure to the air, it becomes lighter in colour. Asiatic ivory, when newly cut, appears more like the African, which has been long exposed to the air, and tends to become yellow by exposure. The African variety has usually a closer texture, works harder, and takes a better polish than the Asiatic. The teeth, however, of both kinds vary greatly in quality and solidity, so that the choice by the purchaser is always hazardous; he is usually guided by the appearance of the rind, which should be smooth and free from cracks, and of the tip, where the rind is to a certain extent worn away, but it is not until the tusk is cut up that flaws, cracks, and imperfections can be detected. It occasionally happens that the musket ball of the huntsman, which failed to kill the animal, has become embedded in the tusk, and encrusted with a fresh deposit of bony matter, to the great injury of the ivory. Gold and silver bullets have on a few rare occasions been found, showing that the shot was fired by royal hands.

The tusk or tooth, as it is generally called, is usually solid for about half its length, circular or elliptical in section, and free from vessels or pores such as occur in bone; and although containing a large proportion of lime, it has none of the harsh, meagre character of bone, but admits of exquisite smoothness, and displays all the niceties of the turners' and carvers' art in the slenderest proportions.

Much judgment is required in cutting up a tooth, so as to apportion the various parts with economy. The ivory saw is a blade from 15 to 30 inches long, and about the fourtieth of an inch thick, set in a steel frame, and tightly stretched, so as to make a very straight cut. The outside strips or spills, as they are called, are used for the handles of penknives; the scraps are burnt for ivory black; and the clean shavings and saw-dust are used for making jelly or size. The mode of cutting up the tooth must of course depend on the uses to which it is to be applied, but in any case it is desirable to mark out the tooth in some way beforehand, and also to pencil out the end of the piece before the saw is used longitudinally. Billiard balls are usually turned out of teeth of nearly the same diameter, called ball-teeth, the object being to prevent the unequal shrinking of the ivory in drying. The ivory shrinks in the direction of the length of the tooth, as well as in the width, but much less in the former than in the latter direction. A billiard ball will vary in its diameters, if used in a room of different temperature to that in which the ivory was kept, so that it is usual to turn the balls roughly to shape, and keep them for some months in the billiard room before the turning is completed. Ivory ornaments should not be kept on a chimney piece which is much exposed to the heat of the fire. The slices of ivory cut for the keys of pianofortes are placed for a few hours in an oven at a temperature sufficient to evaporate the oil, by which means they are not only whitened, but sufficiently shrunk to be at once glued to the wooden bodies of the keys. The action of the atmospheric moisture, in varying the length of ivory, is considerable, so much so that ivory drawing-scales are not to be relied on in accurate work. Scales of box-wood and lance-wood are much to be preferred.

The thin plates of ivory used for the keys of pianofortes, inlaying, and similar work, are usually cut from the solid block by means of a fine feather-edge veneer saw, in which case there is a considerable amount of saw-dust. The Russian method of cutting veneers spirally from a cylinder of wood, with a knife of the same length, has been adopted for ivory, although in such case a reciprocating saw is used, and ivory-dust produced. Mons. Pape of Paris furnishes veneers of this kind as large as 30 by 150 inches, and he has veneered a pianoforte entirely with ivory. We are not disposed to regard the result as pleasing or useful. In the United States department of the Great Exhibition was an ivory veneer 40 feet in length by 12 inches in width. Artificial or fictitious ivory is prepared in France in thin plates or veneers, and it has the appearance of a dull, opaque cement. A superior substance was exhibited by Mr Henry Brown in the Great Exhibition, under the name of British ivory. A prize medal was awarded to the inventor, "on account of its being likely to turn out a valuable and important discovery, and in many cases calculated to supersede real ivory." The vegetable ivory already referred to is a good substitute for ivory where appearance and durability are not of importance: it soon tarnishes, and wears rapidly if exposed to much friction.

Ivory may be dyed of various colours. The red of billiard balls may be given by dipping the ivory for a short time into a mordant of nitro-muriate of tin, and then plunging it into a bath of Brazil wood, cochineal, or a mixture of those substances. A scarlet tint is produced by lac dye, and if the scarlet ivory be plunged into a solution of potash, it changes to cherry red. A yellow die may be pro- duced by giving the ivory the tin mordant, and digesting it at a gentle heat in a clear decoction of fustic. A black dye may be given by boiling the ivory in a strained decoction of logwood, and then steeping it in a solution of red sulphate, or red acetate of iron. In dyeing ivory the surface should not be polished until the dye is set. As soon as the ivory is taken out of the hot-dye bath it should be plunged in cold water to prevent the chance of cracking; and should the dye appear in dark spots or patches, they should be rubbed down with chalk, and the ivory be dyed once more.

When ivory has become discoloured it is not easy to bleach it. Some recommend that it be scrubbed with Trent sand and water, but this is evidently inapplicable to works delicately carved by hand or at the lathe. Articles in ivory, which have suffered partial decomposition by exposure to air and moisture, have been restored by boiling them in a solution of gelatine. Such was the case with some works in ivory sent from Nineveh to this country by Mr Layard. They were in a state of decomposition, and the method of restoring them was suggested by Professor Owen. An attempt has been made to render ivory flexible by removing a portion of its lime by the action of hydrochloric acid. This plan has been adopted by a surgical-instrument maker, M. Charriere of Paris, who first gives the pieces of ivory their required form and polish, then steeps them in dilute acid, until they have become supple and elastic, after which they are taken out and dried, when they again become hard, but the flexibility can be restored by surrounding the ivory instruments with wet linen, or by placing wet sponge in the cavities of the pieces.

The art of working in ivory belongs to the turner, to the carver, to the inlayer, and other artists, and does not call for special notice here.

**IVORY-BLACK.** When bones are subjected to destructive distillation in close vessels, the substance of the bone is resolved into its constituent elements, which recombine and form new compounds. Some of these escape in the form of vapour or gas, such as carbonic acid and various hydro-carbons. Water holding carbonate of ammonia in solution is also formed, and an oil, which is collected and used for feeding the lamps, which, being burnt in close chambers, with a very limited access of air, the oxygen of which is just sufficient to combine with the hydrogen of the oil, but not sufficient to form any compound with the carbon, the results of the combustion are water and carbon, in a minutely divided form, which collects on the walls of the chamber, and forms lamp-black. In the distillation of bones nitrate of ammonia and sulphate of soda are formed towards the end of the process; the latter is separated by solution and crystallization, and the former by sublimation. When all the volatile products are separated, the mass which remains in the retort, consists of the earthy and saline portions of the bone, blackened by the carbon of the animal matter, and forming what is called ivory-black, bone-black, or animal charcoal. In this condition, from its strong attraction for animal colouring matters, it is employed in the refining of sugar for bleaching the syrup, and in other arts for similar purposes. If the bone-black be exposed to a red heat, with access of air, the carbon passes off as carbonic acid, and the bone-ash is left of a light colour, which ash is useful in making cupsels for the assayer; it is also a good polishing powder for plate, and is the raw material for the manufacture of phosphorus.

When the material for the production of bone-black consists of ivory dust or shavings, the calcined matter is called ivory-black; it is ground and levigated on a porphyry slab, and produces a beautiful velvety black, which forms the chief ingredient in the ink of the copper-plate printer. Ivory black may be prepared on a small scale in a covered crucible at an ordinary fire.

(C.T.)

JAMES,** the most considerable British mathematician of his day, was born at Dundee in 1765, spent the most active period of his life as professor at the Military College of Marlow (afterwards removed to Sandhurst), and died Sept. 21, 1843, at the age of seventy-seven. (See Sixth Preliminary Dissertation, by Professor J. D. Forbes, § 103.)