cany, caused experiments to be made at Florence, for the purpose of ascertaining the behaviour of the diamond when exposed to great heat. The diamond was placed in the focus of a burning-mirror, and the operators were not a little astonished to see it gradually disappear, and at length become totally dissipated by the heat, leaving no trace behind. This experiment was subsequently repeated in Vienna by the Emperor Francis I., who applied the heat of a furnace. The diamond was destroyed in the same way. D'Arcet, Rouelle, Maquer, and other French philosophers, then began to experiment upon it, and on the 26th of July 1771, a fine diamond was burnt in Maquer's laboratory, and the extraordinary character of this fact was the talk of all the learned and unlearned of the Parisian world. That the diamond disappeared under intense heat was incontestable, but what became of it no one knew; opinions differed as to whether it became volatilised or burnt away, or was split up into invisible particles. It happened, however, that a celebrated jeweller in Paris, named Le Blanc, denied the destructibility of the diamond by fire, in the teeth of the authority of the learned, asserting that he had frequently exposed diamonds to a strong fire to purify them from certain spots, and that they had never suffered in the least from this; and, adventuring a new experiment, he enclosed a diamond in a mixture of charcoal and chalk in a crucible, and placed it on the fire, convinced that it would come out again uninjured. The Academicians D'Arcet and Rouelle also devoted some diamonds, and these having vanished after three hours' exposure to the fire, Le Blanc's crucible was also opened, when, to his great astonishment, and to the

triumph of the learned, his diamond likewise had disappeared. But the triumph did not endure long; for, on another opportunity, in which the renowned Lavoisier conducted the experiments, another jeweller, Maillard, came, "with a zeal," says Lavoisier, "truly worthy of the gratitude of the learned," and delivered up three diamonds to the torture of the furnace, closing them up well, however, in his own manner, in charcoal powder, in the bowl of a clay pipe. A tremendous heat was applied to them, and when Maillard's pipe-head was opened, there lay the diamonds uninjured in their charcoal powder."

The cutting of diamonds is designed to add to their lustre, and to multiply their faces. It is done by their own dust, by means of revolving circular wheels. This process was discovered by De Berquem, of Bruges, as early as the year 1456. Some time ago, experiments, of a peculiarly interesting kind, were made on the celebrated Koh-i-noor, by Professor Tennant and Sir David Brewster, with the view of giving greater brilliancy to that precious gem.

"Rough diamonds, fit for cutting, are sold at £1, 13s. 4d. to £2 the carat. A carat is rather more than three grains, and 156 carats equal one ounce troy. But if the stones are above one carat, the square of the weight is multiplied by the price of a single carat; so that, for example, a rough stone of three carats costs 3 × 3 × £2, or £18. It is similar with cut diamonds, and at present (1850) the purest brilliants of one carat fetch more than £8, a brilliant of two carats 2 × 2 × £8, or £32. When stones are over eight or ten carats, however, this is altered, so that they are often valued still more highly. Diamonds

of a quarter of an ounce weight are extraordinarily costly, but still larger are met with; and one of the largest known is that of the Rajah of Mattun, in Borneo, which weighs nearly two ounces and a half; that of the Sultan of Turkey weighs two ounces; one in the Russian sceptre more than an ounce and a quarter. The greatest diameter of the last is one inch, the thickness ten lines. The Empress Catherine II. purchased it, in the year 1772, from Amsterdam, and for it was paid £75,000, and an annuity of £650. Diamonds weighing an ounce exist also in the French and Austrian regalia. One of the most perfect is the French, known as the Pitt or Regent diamond. It was bought for Louis XV., from an Englishman named Pitt, for the sum of £135,000 sterling, but has been valued at half a million. One of the stones most renowned in the East is the Koh-i-noor, or Mountain of Light, now in possession of the Queen. It came from Golconda to Persia, and, while uncut, weighed more than five ounces, but now, polished, only about two ounces. It is valued at more than two millions of pounds sterling. If we took only the common mode of estimating the value, a perfect brilliant weighing half a pound would be worth £20,000,000."

COAL.

AMONGST the numerous phenomena, registering the earth's mutations, which have been developed to man by the aid of geological science, none possess greater interest, or afford more subjects for contemplation, than those connected with the extensive coal-formations of Great Britain.

If we cast our eyes over a geological map of our island, we find sundry black patches of colour indicating the localities in which coal is found. First, there is the Scotch coal-field extending from sea to sea, embracing Edinburgh and Glasgow; then the valuable district of Northumberland and Durham, followed by the extensive coal-fields of the West Riding of Yorkshire and the equally important ones of Lancashire and Staffordshire. Smaller patches are seen scattered through Leicester and the counties westward. Glamorganshire presents an extensive coal district and in Pembrokeshire we have another-producing a coal known as anthracite. With the Forest of Dean and the Bristol coal-field in Gloucestershire, the coal-formations terminate southward. These form a total area of 4068 square miles, and include at least 3000 working collieries. The coal-fields of Ireland embrace areas equal to 2227 square miles; but these are not developed with the same amount of industry and skill as those of England and Scotland.

Coal occurs in beds these should be especially distin

guished from the veins or lodes in which the metalliferous deposits are formed. Generally the coal-formations give evidence of their having been produced in a nearly horizontal position. This has not unfrequently been altered considerably by movements subsequently to the deposition of the matter forming the coal, and therefore now in many of our mines the workings on the coal-beds are at a high inclination.

If the reader will imagine an immense lake, or a large delta, in which a series of deposits, differing in their character, has been formed, a section of these will give the appearances which present themselves in a section of our coal-measures. To adopt a yet more simple illustration : clay and coal-dust, we will suppose, are thoroughly diffused through the water contained in a deep vessel. According to the laws of gravity the clay will subside first, and then the lighter coal-dust, leaving the water eventually clear. More clay and coal-dust are mixed with the water; we shall then have another deposit similar to the first, and this we may repeat any number of times. Our deposited

mass would at last present a section such as that shewn in the adjoining figure, the thicknesses of the bands of coal and clay depending upon the quantities of each held in suspension by the water.

This is merely an indication of the conditions of bedding, and is not to be considered as an explanation of the actual formation of coal. Chemical analysis and microscopical examination equally prove the vegetable origin of coal.