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or filled with water. It is also remarkable, that the glasses broke upon having their bottoms slightly rubbed with the finger, though some of them did not fly till half an hour after the rubbing. If the glasses are every where extremely thin, they do not break in these circumstances. Some have pretended to account for these phanomena, by saying, that the bodies dropped into the vessels cause a concussion which is stronger than the cohesive force of the glass, and consequently that a rupture must ensue. But why does not a ball of iron, gold, silver, or copper, which are perhaps a thousand times heavier than flint, produce the same effect? It is because they are not elastic. But surely iron is more elastic than the end of one's finger. Mr. Euler has endeavoured to account for these appearances from his principles of percussion. He thinks that this experiment entirely overthrows the opinion of those who measure the force of percussion by the vis viva, or absolute apparent strength of the stroke. According to his principles, the great hardness and angular figure of the flint, which makes the space of contact with the glass extremely small, ought to cause an impression on the glass vastly greater than lead, or any other metal; and this may account for the flint's break ing the vessel, though the bullet, even falling from a considerable height, does no damage. Hollow cups made of green bottle-glass, some of them three inches thick at the bottom, were instantly broken by a shiver of flint, weighing about two grains, though they had resisted the shock of a musket-ball from the height of three feel. That Mr. Euler's theory cannot be conclusive any more than the other, must appear evident from a very slight consideration. It is not by angular bodies alone that the glasses are broken. The marbles with which children play are round, and yet they have the same effect with the angular flint. Besides, if it was the mere force of percussion which broke the glasses, undoubtedly the fracture would always take place at the very instant of the stroke; but we have seen, that this did not happen sometimes till a very considerable space of time had elapsed. 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 power becomes at last too weak to resist them. There can be little doubt that the fluid just now mentioned is that of electricity. It is known to exist in glass in very great quantity; and it also is known to be capable of breaking glasses, even when annealed with the greatest care, if put into too violent a 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 fluid by friction or otherwise. This is evidently the case when it is broken by rubbing with the finger; but why it should also break by the mere contact of flint and the other bodies above mentioned, has not yet been satisfactorily accounted for.

A most remarkable phenomenon also is produced in glass tubes placed in certain circumstances. When these 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 a nearly 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 perfectly upright posture, the less will the motion be either way. If the tube is placed horizontally on a glass plane, the fragment, for instance, of 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 recede from the fire, and move a little up hill when the plane inclines towards the fire. These experiments are recorded in the Philosophical Transactions. They succeeded best with tubes about 20 or 22 inches long, which had in each end a pretty strong pin fixed in cork for an axis.

The reason given for these phænomena is the swelling of the tubes towards the fire by the heat, which is known to expand all bodies. For, say the adopters of this hypothesis, granting the existence of such a swelling, gravity must pull the tube down when supported near its extremities; and a fresh part being exposed to the fire, it must also swell out and fall down, and so on. But, without going farther in the explanation of this hypothesis, it may be here remarked, that the fundamental principle on which it proceeds is false: for though fire indeed makes bodies expand, it does not increase them in weight; and therefore the sides of the tube, though one of them is expanded by the fire, must still remain in equilibrio; and hence we must conclude, that the cause of these phænomena remain yet to be discovered.

4. Glass is less dilatable by heat than metalline substances, and solid glass sticks are less dilatable than tubes. This was first discovered by Col. Roy (see Phil. Trans. vol. lxvii. p. 663), in making experiments in order to reduce barometers to a greater degree of exactness than hath hitherto been found practicable; and since his experiments were made, one of the tubes 18 inches long, being compared with a solid glass rod of the same length, the former was found by a pyrometer to expand four times as much as the other, in a heat approaching to that of boiling oil. On account of the general quality which glass has of expanding less than metal, M. de Luc recommends it to be used in pendulums: and he says it has also this good quality, that its expansions are always equable and proportioned to the degrees of heat; a quality which is not to be found in any other substance yet known.

5. Glass appears to be more fit for the condensation of vapours than metallic substances. An open glass filled with water, in the summer time, will gather drops of water on the outside, just as far as the water in the inside reaches; and a person's breath blown on it manifestly moistens it. Glass also becomes moist with dew, when metals do not.

6. A drinking-glass partly filled with water, and rubbed on the brim with a wet finger, yields musical notes, higher or lower as the glass is more or less full, and will make the liquor frisk and leap about. See ARMONICA.

7. Glass is possessed of extraordinary electrical virtues. See ELECTRICITY, passim.

GLASS (Manufacture of). Glass is made from

sand, flints, spar, or some other silicious matters. White sand is the substance in the most repute at present, as it requires no preparation for coarse goods; and for the finest, washing in fair water is sufficient: whereas flints require a tedious process of calcination, and after that to be pulverized. Many other substances may be used for experiment; though sand only is employed in the manufactory.

It is also necessary that the silicious matter should be fused in contact with something called a flux. The substances proper for this purpose are lead, borax, arsenic, nitre, or any alkaline matter. The lead is used in the state of red lead, and the alkalies are soda, pearl-ashes, sea salt, and wood-ashes. When red lead is used alone, it gives the glass a yellow cast, and requires the addition of nitre to correct it. Arsenic, in the same manner, if used in excess, is apt to render the glass milky. For a perfectly transparent glass, the pearl-ashes are found much superior to lead; perhaps better than any other flux, except it be borax, which is too expensive to be used, except for experiments, or for the best looking-glasses.

The materials for making glass must first be reduced to powder, which is done in mortars or by horse-mills. After sifting out the coarse parts, the proper proportions of silex and flux are mixed together and put into the calciniog furnace, where they are kept in a moderate heat for five or six hours, being frequently stirred about during the process. When taken out the matter is called frit. Frit is easily converted into glass by only pounding it, and vitrifying it in the melting pots of this glass furnace: but in making fine glass it will sometimes require a small addition of flux to the frit to correct any fault. For, as the flux is the most expensive article, the manufacturer will rather put too little at first than otherwise, as he can remedy this defect in the melting pot. The heat in the furnace must be kept up until the glass is brought to a state of perfect fusion; and during this process any scum which arises must be removed by ladles. When the glass is perfectly melted, the glass-blowers commence their operations.

The following composition of the ingredients for glass are extracted from the Handmaid to the Arts:

"

For the best flint-glass, 120lbs. of white sand. 50lbs, of red lead, 40lbs. of the best pearlashes, 20lbs. of nitre, and five ounces of magnesia; if a pound or two of arsenic be added, the composition will fuse much quicker, and with a lower temperature.

"For a cheaper flint-glass, 120lbs. white sand, 35lbs. of pearl-ashes, 40lbs. red lead, 13lbs. of nitre, six pounds of arsenic, and four ounces of magnesia.

This requires a long heating to make clear glass; and the heat should be brought on gradually, or the arsenic is in danger of subliming before the fusion commences. A still cheaper composition is made by omitting the arsenic in the foregoing, and substituting common sea salt.

"For the best German crystal glass, 120lbs. of calcined flints or white sand, the best pearl-ashes 70lbs., saltpetre 10lbs., arsenic half a pound, and five ounces of magnesia. Or, a cheaper composition for the same purpose is, 120lbs. of sand or flints, 46lbs. of pearl-ashes, seven pounds of nitre, six pounds of arsenic, and five ounces of magnesia. This will require a long continuauce in the

furnace; as do all others where much of the arsenic is employed.

"For looking-glass plates, washed white sand 60lbs., purified pearl-ashes 25lbs., nitre 15lbs., and seven pounds of borax. If properly managed, this glass will be colourless. But if it should be tinged by accident, a trifling quantity of arsenic, and an equal quantity of magnesia, will correct it; an ounce of each may be tried first, and the quantity increased if necessary.

"The ingredients for the best crown-glass must be prepared in the same manner as for lookingglasses, and mixed in the following proportions: 60ibs. of white sand, 30lbs. of pearl-ashes, fand 15lbs. of nitre, borax a pound, and half a pound of arsenic.

"The composition for common green windowglass is 120lbs. of white sand, 30lbs. of unpurified pearl-ashes,wood-ashes well burnt and sifted 60lbs. common salt 20lbs., and five pounds of arsenic.

"Common green bottle-glass is made from 200lbs. of wood-ashes, and 1001bs. of sand; or 170lbs, of ashes, 100lbs. of sand, and 50lbs. of the lava of an iron-furnace: these materials must be well mixed."

The materials employed in the manufactory of glass, are by chemists reduced to three classes, namely, alkalies, earths, and metallic oxyds.

The fixed alkalies may be employed indiffer ently; but soda is preferred in this country. The soda of commerce is usually mixed with common salt, and combined with carbonic acid. It is proper to purify it from both of these foreign bodies before using it. This, however, is seldom done.

The earths are silica, lime, and sometimes a little alumina. Silica constitutes the basis of glass. It is employed in the state of fine sand or flints; and sometimes, for making very fine glass, rock crystal is employed. When sand is used, it ought if possible to be perfectly white; for when if is coloured with metallic oxyds, the transparency of the glass is injured. Such sand can only be employed for very coarse glasses. It is necessary to free the sand from all the loose earthy particles with which it may be mixed, which is done by washing it well with water.

Lime renders glass less brittle, and enables it to withstand better the action of the atmosphere. It ought in no case to exceed the twentieth part of the silica employed, otherwise it corrodes the glass pots. This indeed may be prevented by throwing a little clay into the melted glass; but in that case a green glass only is obtained.

The metallic oxyds employed are the red oyyd of lead or litharge, and the white oxyd of arsenic. The red oxyd of lead, when added in sufficient quantity, enters into fusion with silica, and forms a glass without the addition of any other ingre dient. Five parts of minium and two of silica form a glass of an orange-colour and full of striæ. Its specific gravity is five. The red oxyd of lead renders glass less brittle and more fusible; but, when added beyond a certain proportion, it injures the transparency and the whiteness of glass.

The white oxyd of arsenic answers the same purposes with that of lead, but on account of its poisonous qualities it is seldom used. It is cu tomary to add a little nitre to the white oxyd of arsenic, to prevent the heat from reviving it, and rendering it volatile. When added beyond a cer tain proportion, it renders glass opaque and malay like the dial-plate of a watch. When any com

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