Saxony, Spain, Jersey, Massachusetts, and Greenland. It is usually associated with the older crystalline rocks, such as granular limestones, gneiss, mica-schists, and talc-schists, but occurs also in rolled masses and detached blocks. "The emery generally used in this country," says Mr Bristow, "is found in the island of Naxos, where it occurs in large blocks, imbedded in a red soil, and sometimes in whole marble. These blocks are so abundant, that, notwithstanding the immense quantities carried off, it is not yet requisite to quarry the rock itself. This substance is of so much value in the arts, that an English merchant found it advantageous to obtain a monopoly of it from the Greek Government; in consequence of which its price in this country has been greatly advanced, and at one time was as high as £30 per ton." At present a large proportion of the emery used both in England and America is obtained from near Ephesus, and shipped at Smyrna.

In its preparation the original blocks are first broken with heavy hammers, then crushed with steel-headed stampers driven by steam-power, and afterwards passed through various sieves, which assort it into different degrees of fineness (about twenty, it is said), such as "flour," "corn," "grinding-emery," &c. Much of it is used in the manufacture of emery-paper and emery-cloth, in both of which articles it is often largely adulterated with triturated iron-slag, garnet-rock, and other hard substances. Indeed, the so-called red emery is merely the massive garnet-rock of Norway, stamped, crushed, and reduced to powder-suitable enough for certain purposes, but inferior in hardness, and consequently in abrasive power, to the real emery. In this sifted and prepared state it is extensively employed in grinding and polishing by plate-glass manufacturers, by lapidaries, by marble and granite cutters, and by all the workers in metals.

Of recent years it has been formed into solid grinding-wheels and cutting-discs either by the silicate of lime process of Mr Ransome (see Chap. VI.), or by the Tanite process of America. In either case the emery particles are cemented into solid discs of unrivalled cutting power-their efficiency depending partly on the hardness of the mineral, and partly on the speed at which they are driven-their toughness and compactness permitting them to be driven at a rate which would be destructive to any natural stones. In hardness it varies from 7 to 9; in specific gravity from 3.95 to 4.5; and in abrasive power (sapphire being 100) from 45 to 57. The following are analyses by J. Lawrence Smith:

Diamond, in addition to its value as a precious stone, is employed for engraving and cutting glass; in splinters, for drilling; and, reduced to powder, for polishing and cutting other gems. Its specific gravity is about 3.5, and its hardness 10; and being thus the hardest of minerals, all other mineral substances must yield to its cutting and abrasive powers. Diamond-powder being worth £50 per ounce, it is too expensive to be used alone, and is therefore generally mixed with emery, and applied to the wheel with oil. Chemically, diamond is carbon or charcoal in its pure and crystallised form; geologically, nothing is known of its origin and formation. For its mode of occurrence in India, Brazil, Australia, the Urals, California, and Cape Colony, the reader is referred to the section on 66 Gems and Precious Stones ;" and under this head it may be enough to mention that it is only the smaller, the flawed, and the inferior in colour, that are reduced to powder for cutting and polishing purposes. These inferior sorts are usually known as Bort or Boort and Carbonado, being greyish-white, or even of darker colours, and occurring in irregular, twisted-looking, and mammillary fragments. We have already alluded to the use of diamond-points in the dressing of burr millstones; more recently they have been employed as a drill for rock-boring (Beaumont's Diamond Drill); and our American friends are now experimenting with diamond saws— that is, blades of iron armed with diamonds-for the sawing or cutting up of rock-blocks, a process at present tediously accomplished by iron and quartz-sand.

V. WHETSTONES AND HONES.

For whetting or sharpening the smaller edge - tools and implements, a great variety of stones are in use; and in the majority of these, quartz or silica, in coarser or finer states of dissemination, is the reducing material. From fine-grained sandstones to the most intimate admixture of impalpable silica, batts, ragstones, whetstones, and hones, are in constant requisition, some being used dry, some with water, and others with

oil. It is very difficult to arrange these whetstones into anything like scientific order, partly because the same material is found in several formations, and partly because the same substance receives different names in different localities. Beginning with the more common and coarser-grained, we may notice them in the following order :-

Batts.

Batts or Sandstones of keen grain and toughish consistency are found in many districts, and are used as whetters for scythes, mowers, hedge-bills, knives, and other similar implements not requiring a smooth or delicate edge. The Devonshire batts, from the greensands of the Blackdown Hills-the Yorkshire, from the millstone grit and Gannister beds of the coalformation-and the Lomond, from the lower carboniferous sandstones of Fifeshire,-have good reputations; but suitable sandstones might be found in many other places if skilled hands would only commence the industry. A good batt should have a keen bite, be equal in texture, and not be apt to "glaze" or wear too smooth in the surface. They are cut of all sizes, from five to fifteen inches, and fashioned in all forms -square, cylindrical, spindle-shaped, and oblong.

Ragstones.

What are known as Norway, Russian, and Scotch Ragstones, are merely tough and highly silicious portions of mica-schist, often with a slight twist in the long direction. They are obtained from the metamorphic or crystalline schists, have a keen severe bite, and are well adapted for rapid reduction. They are generally sold in their natural state, or with very little dressing, and can be used dry, with water, or with oil. The Russian sorts are usually softer and more micaceous than the Scotch and Norwegian.

Hones.

Hones, or Oilstones, as they are generally termed by the workmen using them, are obtained from the varieties of the metamorphic schists and slates which are sufficiently compact, and in which the particles of quartz are extremely minute and regularly disseminated, so as to give them a uniform consistence. Among the varieties most prized are the Turkey Oilstones, obtained from the interior of Asia Minor, of a yellowish or bluish colour, of very close grain, often flawed and cracked, and therefore rather brittle, and requiring to be backed with slate or set in wood; the German razor-hone or Novaculite,

from the slate - hills in the neighbourhood of Ratisbon, of keener bite and tougher texture; the Arkansas Oilstone, from North America (largely prepared at Keswick), consisting of a very fine white amorphous silica, of excellent bite, but rather brittle and expensive;* the Welsh Slate or Oilstone, a tough, fine-grained, silicious slate, but of rather unequal texture; the Charley Forest Oilstone, a strong and serviceable whetstone, from Leicestershire; and the Water-of-Ayr or Snake stone, apparently an altered shale of greyish mottled colour, rather soft in texture, and chiefly used for polishing copper-plates and the mouldings of ornamental marbles. As the name implies, most of these whetters are used with oil, and are chiefly employed in giving the last or finished edge to the finer-cutting tools and implements. Many of them being rare and expensive (from three to five shillings per pound), they are used in small blocks, and backed with slate or set in wooden frames, to give them the necessary strength and solidity.

VI. BURNISHERS.

Though tools of hard-tempered and highly-polished steel are mostly used for metal-burnishing, other burnishers for foils (gold-leaf, silver-leaf, and the like) are obtained from the mineral kingdom. These are chiefly agates, bloodstones, carnelians, and jaspers-all hard, tough, and compact varieties of silica, obtained from geodes and veins in the amygdaloid and other igneous rocks; but as these substances are likewise used as gems and ornamental stones, their geological history will be treated more fully under the section "Gems and Precious Stones," to which the reader is referred. Burnishers of highlytempered steel are also largely used; but for certain purposes, agates and carnelians, though much more expensive, are always preferred.

All the substances noticed in the preceding sections, whether millstones, grindstones, whetstones, or polishers, are of infinite use in the Arts and Manufactures, their utility depend

* An analysis of this whetstone, by C. E. Waite, shows that it consists of—

ing upon their hardness as compared with that of the substances to which they are applied. The majority of rocks, wrought metals, glass, and the like, do not exceed 5 in hardness; while the silicas, emeries, and diamonds range from 7 to 10, and are thus capable of cutting, abrading, and polishing the softer materials. The rapidity with which the abrading substance is applied has also a certain effect; and in this way even diamond itself can be cut and polished by its own powder. Considering the necessity for edge-tools of high temperfor smooth surfaces to diminish friction in machinery, for true forms to increase the power of optical glasses, and for polish and lustre to produce ornamental effect-these substances are of great value in the arts; and any addition, geologically speaking, that can be made to their number and efficiency, will be hailed as a boon. The whole matter is a question of hardness, of abundance, and facility of application; and the field geologist can have little difficulty in determining qualities in any new mineral that may be discovered in the course of his surveys. Important as may be the collecting of facts, they are shorn of half their significance when not made to administer to the requirements of society and civilisation.