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nealed glass have been discovered, which have the remarkable property of resisting very hard strokes given from with: out, though they shiver to pieces by the shocks received from the fall of very light and minute bodies dropped into their cavities. These glasses may be made of any shape; all that is necessary in their formation is, that the bottom should be thicker than the sides. The thicker the bottom is, the easier do the glasses break. A glass having the bottom three inches in thickness, flies with as much ease as the thinner glass would do. Some of these vessels have been struck by a mallet with force sufficient to drive a nail into wood tolerably hard, and have not been broken: they have also resisted the shock of heavy bodies let fall into their cavities, from the height of two or three feet, such as musketballs, pieces of iron, and other metals, jasper wood, bone, and so forth. But this is not surprising, because other glasses will do the same, but the wonder is, that taking a shiver of fint of the size of a small pea, and letting it fall into the glass, only from the height of three inches, in about two minutes the glass flies, and sometimes at the moment of the shock; a bit of flint no larger than a grain of sand dropped into several glasses successively, though it did not immediately break them, yet when set by, they all flew in less than three quarters of an hour.

“Sapphire, diamonds, porcelain, hard-tempered steel, pearls, and marbles, such as boys play with, produce the same extraordinary effect. It is also remarkable, that the glasses broke upon having the bottoms rubbed with the finger, though some of them did not break till half an hour afterwards. If the glasses are every where thin alike, they do not break under these circumstances.

EDWARD." How very extraordinary. Can you account for this curious phenomenon, Sir ?

DR.WALKER.-" Not very satisfactorily. Some have pretended to account for these circumstances, by attributing them to the concussion produced, by dropping the articles into the cavities of the glasses, which being stronger than the cohesive powers of the glass, a rupture must necessarily ensue ; but this reason is by no means conclusive, unless they could tell us what principle it is which makes the small piece of flint, weighing about two grains, more powerfully concussive than iron, copper, gold, and so forth, which are a thousand times heavier than the

Aint. Perhaps the most plausible cause may be that of electricity; for if the effect were produced by the mere force of percussion, the fracture would take place instantly, but that is not always the case. It is evident, therefore, that this effect is occasioned by the putting in motion some subtile fluid with which the substance of the glass is filled, and that the motions of this fluid, when once excited in a particular part of the glass, soon propagate themselves through the whole, or greatest part of it, by which means the cohesive powers become too weak to resist them. There can be little doubt but that this fluid is electricity. Glass is known to contain a large quantity of this powerful fluid, which possesses the power of breaking glasses, even when annealed with the greatest care ; it put into too violent motion. Probably the cooling of glass hastily, may make it more electric than is consistent with its cohesive power, so that it is broken by the least increase of motion in the electric Auid, by friction or otherwise. This is evidently the case, when it is broken by the touch of the finger ; but why it should also break by the mere contact of the flint, and the other bodies I have mentioned, has not yet been satisfactorily accounted for.

“ I can tell you of another phenomenon equally remarkable, and which has never yet been explained, neither does it appear probable that it should. When glass tubes are laid before a fire, in an horizontal position, having their extremities properly supported, they acquire a rotatory motion round their axis, and also a progressive motion towards the fire, even when their supports are declining from the fire, so that the tubes will move a little way up hill towards the fire: When the tubes are placed in nearly an upright posture, leaning to the right hand, the motion will be from east to west, but if they lean to the left hand, their motion will be from west to east; and the nearer they are placed to the upright posture, the less will the motion be either way.

If the tube be placed horizontally on a glass plane, the fragment for instance of a coach-window glass ; instead of moving towards the fire, it will move from it, and about its axis in a contrary direction to what it had done before, nay it will even recede from the fire, and move a little up hill when the plane inclines towards the fire. Now, these are most extraordinary and most unaccountable experiments which are all recorded in the Philosophical Transactions. The philosophers who made these experiments, succeeded.

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best with tubes, about twenty or two and twenty inches. long, which had in each end a pretty strong pin, fixed in a cork for an axis."

EDWARD.—“How exceedingly curious!"

Dr.WALKER. “Having thus discussed its wonders, we will proceed to a more minute description of its formation. The materials employed in the manufactory of glass, are by chea. mists reduced to three classes, viz. alkalies, earths, and metallic oxydes. Alkalies possess the following properties : they have the power of converting a vegetable blue to a green colour; manifest a hot and caustic taste; and are soluble in water. They are divided into classes fixed and volatile, and are again subdivided into vegetable and mineral; the former being the production of vegetables burut in the open air, and the latter has sometimes been found in a natural state in the earth. Marine plants however, furnish the largest quantity of this valuable article. There are but three alkalies known at present-Potash, Soda, and Ammonia. Potash and soda are termed fixed alkalies, but ammonia is a volatile alkali; when dissolved in caloric in the form of gas, it: has a pungent and suffocating smell. Ammonia is procured by burning animal substances. Formerly it was imported in large quantities from Egypt, as contained in sal ammoniac. This was prepared from camel's dung, but it is now obtainedfrom a distillation of bones, and is called hartshorn. These digressions will arise from the description of all chemical processes. As you wish thoroughly to comprehend the subject under discussion, I shall therefore lrave no scruples in: making them--they are by far too curious to weary your attention.”

EDWARD. -" Indeed they are, Sir." }

Dr.WALKER.-" The fixed alkalies may be employed indifferently, but soda is preferred in this country. The soda of commerce is usually mixed with common salt and car.. bonic acid, from both of which it must be purified, before it can be used in making glass. The earths are silicia, the basis of flints, lime, and a little alumina, the basis of clay. Rock crystal is sometimes used when the glass is to be particularly fine.

" The metallic oxydes employed, are the red oxyde of lead called minium, or litharge, and the white oxyde of arsenic. The oxyde of lead, when added in sufficient quantities, enters into fusion with lead, and forms a glass without the addition of

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any other ingredient. This oxyde renders glass less brittle and more fusible, but if added in too large quantities, it injures its transparency. The oxyde of arsenic is not much used on account of its poisonous qualities."

EDWARD.-" How do they produce the beautiful coloured glasses."

DR.WALKER. -“* Blue glass is formed by means of oxyde of cobalt; cobalt is a fossil, of the morcasite species, containing a large quantity of arsenic.

“Green glass, by the oxyde of iron, or copper.

“ Violet glass, by oxyde of manganese. Manganese is a bril. liant metal, of a dark grey colour, of considerable hardness, and difficult fusibility. It is very brittle, and when in powder it has the peculiar property of being attracted by tlie magnet.

The oxydes of manganese are also used in blacking, and in purifying glass, as well as colouring it; it is likewise employed in glazing black earthenware. The black oxyde is also much used by chemists, for producing oxygen gas, which by the application of a red heat, it yields in great abundance.

“Red glass, by a mixture of the oxydes of copper and iron.
" Purple glass, by the purple oxyde of gold.
“White glass, by the oxyde of arsenic and zinc: and

Yellow glass, by the oxyde of silver, and by com. bustible bodies.

“ These, my dear Edward, are the principal properties and articles used in the composition of glass ; as to the manner of its formation into plates, &c. that you will see at Stourbridge.

“ You will do well to make a memorandum of what I have told

Our travellers having taken some refreshment, were impatient to view the glass-houses, where they were greatly amused with the dexterity, with which the men blew the glass into such various shapes and forms. Those des. tined for watch glasses are blown into a globular form; one globe making many glasses, which are cut by an iron ring.

All glass, except plate glass, is formed by dipping the end. of a very long iron pipe, when red hot, into the boiling glass, and blowing through it, till the bladder of glass is of the size necessary for the purpose to which it is destined, it is then cut up the middle with a pair of shears, if for window glass, or separated by means of cold water, if destined to be of a circular, or any such form.,

you."

Plate-glass is cast; that is to say, the liquid is conveyed from the mouth of the furnace to a large table, on which it is poured, and the excrescencies, or bubbles, are immediately removed by a roller, that is swiftly passed over it; it is then cooled, or annealed in the usual way, by being removed by degrees to the coolest part of the annealing chamber.

SECTION VIII.

DERBYSHIRE.

Our travellers pursued their journey rapidly till they came to Derby, where they made a short stay, previous to their visit to the Peak.

Derby is situated on the west bank of the Derwent, over which it has a fine stone bridge, well built, upon which there was formerly a chapel,. now converted into a dwellinghouse. The celebrated silk mill, erected by Sir Thomas Lambe about the middle of the last century, afforded much entertainment to our travellers. Sir Thomas brought the model from Italy, where he ran great risks in procuring it; but having accomplished his end, he embarked for his native country, bringing with him so great a treasure. There are nearly 100,000 movements turned by a single wheel, any one of which may be stopped independent of the rest. Every time this wheel goes round, which is three times in. every minute, it works 73,728 yards of silk.

The money given by strangers who inspect this mill, is put into a box, which is opened the day after Michaelmas day, and a feast is made for the men, women, and children, who are employed in the works. This is a holiday fondly anticipated by this part of the community. Drest in their best attire, they gaily assemble to partake of a whole roasted ox, and other good fare, which is provided for them. Singing and dancing conclude the evening's amusement, and the town is illuminated. Derby contains also a china manufactory, and the Derbyshire spars are worked into every kind of

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