milky like the dial-plate of a watch. When any combustible body is present, it is usual in some manufactures to add a little white oxyd of arsenic. This supplying oxygen, the combustible is burut, and flies off; while the revived arsenic is at the same time vola. tilized.

There are several kinds of glass adapted to different uses. The best and most beautiful are the flint and the plate glass. These, when well made, are perfectly transparent and colourless, heavy and brilliant. They are composed of fixed alkali, pure siliceous sand, calcined flints, and litharge, in different proportions. The flint-glass contains a large quantity of oxyd of lead, which by certain processes is easily separated. The plate-glass is poured in the melted state upon a table covered with copper. The plate is cast half an inch thick, or more, and is ground down to a proper degree of thinness, and then polished.

Crown-glass, that used for windows, is made without lead, chiefly of fixed alkali fused with silicious sand, to which is added some black oxyd of manganese, which is apt to give the glass a tinge of purple.

Bottle-glass is the coarsest and cheapest kind: into this little or no fixed alkali enters the composition. It consists of an alkaline earth combined with alumina and silica. In this country it is composed of sand and the refuse of the soap boiler, which consists of the lime employed in rendering his alkali caustic, and of the earthy matters with which the alkali was contaminated. The most fusible is flint glass, and the least fusible is bottle glass.

Flint-glass melts at the temperature of 10o Wedgewood; crown. glass at 30°; and bottle-glass at 47°. The specific gravity varies between 2.48 and 3.38.

Glass is often tinged of various colours by mixing with it, while in fusion, some one or other of the metallic oxyds; and on this process, well conducted depends the formation of pastes or facti. tious gems.

Blue glass is formed by means of oxyd of cobalt.
Green, by the oxyd of iron or of copper.

Violet, by oxyd of manganese.

Red, by a mixture of the oxyds of copper and iron.
Purple, by the purple oxyd of gold.

White, by the oxyd of arsenic and of zinc.

Yellow, by the oxyd of silver and by combustible bodies.

Opticians, who employ glass for optical instruments, often complain of the many defects under which it labours. The chief of these are the following:

Streaks. These are waved lines, often visible in glass, which interrupt distinct vision. They are probably owing sometimes to want of complete fusion, which prevents the different materials from combining sufficiently; but in some cases also they may be produced by the workmen lifting up, at two different times, the glass which is to go to the formation of one vessel or instrument.

Tears. These are white specks or knots, occasioned by the vi trified clay of the furnaces, or by the presence of some foreign

salt.

Bubbles. These are air-bubbles which have not been allowed to escape. They indicate want of complete fusion, either from too little alkali, or the application of too little heat.

Cords. These are the asperities on the surface of the glass, in consequence of too little heat.

Glass-blowing.

The art of forming vessels of glass is termed blowing, from its being in agreat measure performed by the operator blowing through an iron tube, and by that means inflating a piece of glass which is heated so as to become soft and exceedingly pliable. By a series of the most simple and dexterous operations, this beautiful material is wrought into the various utensils of elegance and utility, by methods which require but very few tools, and those of the most simple construction.

Watch-glasses are made by first blowing a hollow globe, the proper radius for the glasses; then by touching it with an iron ring. This cracks out a watch-glass in an instant. The same globe will make several glasses.

Window or table-glass is worked nearly in the same manner: the workman blows and manages the metal, so that it extends two or three feet in a cylindrical form. It is then carried to the fire, and the operation of blowing repeated till the metal is stretched to the dimensions required, the side to which the pipe is fixed diminishing gradually till it ends in a pyramidal form; but, in order to bring both ends nearly to the same diameter, while the glass continues flexible, a small portion of hot metal is added to the pipe; the whole is drawn out with a pair of iron pincers, and the same end is cut off with a little cold water as above.

The cylinder thus open at one end is returned to the mouth of the furnace, where it is cut by the aid of cold water, and ripped up through its whole length by a pair of iron shears; after which it is gradually heated on an earthern table, in order to unfold its length, while the workman with another iron tool alternately raises and depresses the two halves of the cylinder: by which process, the one half accommodates itself to the same flat form as the other.

Plate-glass is the last and most valuable kind, and is thus called from its being castin plates or large sheets: it is almost exclusively employed for mirrors or looking-glasses, and for the windows of carriages.

Plate-glass was formerly blown; but that method having been found very inconvenient, casting was invented; namely, the liquid metal is conveyed from the furnace to a large table, on which it is poured, and all excrescences, or bubbles, are immediately removed by a roller that is swiftly passed over it. It is then an. pealed in the manner already referred to.

SECTION 14.

Rupert's Drops. Batavian Tears. Bolognian Phial. THESE are peculiar modifications of glass, for the purpose of de. ception or amusement.

Rupert's Drops, an elegant glass toy, are simply formed by pouring a small solid tump of green bottle glass, when red-hot, into water, by which means the rounded lump assumes gradually a lengthened form, terminating with a fine and nearly capillary tail, at the extremity. This solid lump will bear very considerable violence on the massy end without injury, and is altogether extremely tough; but whenever the smallest portion of the thinner end is broken off, the whole bursts with a smart suap, and instantly crumbles into innumerable fragments as small as fine sand; which, from their very minuteness, and the imperfection of their crystallization, do no other injury to the hand that holds the drop, than that of pro. ducing a slight sting from the sudden concussion.

This curious and extraordinary fragility is obviously owing to some permanent and very strong inequality of pressure; for when the Rupert's drops are heated so red as to be soft, and are let to cool gradually of themselves, and, consequently, to become better annealed, this property of bursting is entirely lost, and, at the same time, the specific gravity of the drop is increased.

These drops are also called, on the continent, Larmes Batavi. ques, or Batavian Tears.

All glass, not regularly annealed, or, in other words, cooled suddenly instead of progressively, has a tendency towards the same frangibility. Thus, in common window glass, if it be properly annealed, the diamond cuts it with moderate ease, making an uniform smooth furrow, at first dark, but gradually opening, and appearing like a bright silver thread: but when the glass is badly annealed, the diamond works with much more difficulty, the cut opens very slowly, and often flies into a different direction, or the glass entirely breaks.

There is another equally curious glass toy, formed upon the same principle, and evincing the same effect, called the Bologna phial. This is simply a phial, of any shape whatever, made of any kind of glass, but much thicker at the bottom than at top, and cooled immediately, without annealing. These being pretty stout, from their thickness will bear a smart blow from a wooden mallet, or any blunt instrument, or the concussion of a leaden bullet drop. ped from a considerable height, without injury: but if any sharp body, however small, such as a large grain of sand, or which is still better, the shiver of a gun-flint, be dropped in from only a few inches height, the bottom cracks all around, just above the thickest part, and drops off. The same effect takes place, if the bottom be slightly scratched with any hard body. When very brittle, if a hard angular substance, as a cut diamond, be dropt in, it will sometimes pass through the bottom, though very thick, with apparently as little resistance as through a spider's web. These glasses, when they have received the first injury, do not always crack immediately, but remain whole, sometimes for a few minutes, sometimes for hours, and then suddenly give way.

[Pantologia, Aikin's Chem. Dict.

CHAP. XIII.

GUNPOWDER.

SECTION 1.

Of the time when gunpowder was first discovered.

THE history of the discovery of gunpowder is involved in much obscurity; the most ancient authors differing from each other in their accounts of this matter, and many of them confounding two distinct inquiries; the discovery of the composition of gunpowder; and the discovery of the means of applying it to the purposes of

war.

Father Kircher* affirms, that without controversy we ought to attribute the invention of gunpowder to Barthold Schwartz, or Barthold the black, a monk of Goslar in Germany, and a profound alchemist. This man having mixed together, with a medical view, nitre, sulphur, and charcoal, a spark accidentally fell upon the mixture, blew up the pot in which it was contained, and caused a dreadful explosion. The monk, astonished at the event, made se. veral repetitions of his experiment, and thereby fully discovered the nature of gunpowder, in the year 1354. Kircher gives us also, out of a very old German book which he professes to have read, a monkish account of the first use which Schwartz made of his gun. powder; he employed it to frighten some robbers from their haunts in the woods.

Sebastian Munster says, that he was well informed by a very eminent physician, that the Danes used guns in naval engagements in the year 1354, and that a chemist, called Schwartz, was the first inventor of them t. Pontanus, the Danish historian, accedes to this opinion.

Polydore Virgil, who died in the year 1555, attributes the dis