Two toys are made of unannealed glass, which, though commonly used for the amusement of children, exhibit phenomena which justly interest the curiosity of the philosopher; we mean Prince Rupert's drops, and the Bologna flask or philosophical phial.

Prince Rupert's Drops are made by letting drops of melted glass fall into cold water: the drop assumes, by that means, an oval form, with a tail or neck resembling a retort. These drops are said to have been first invented by Prince Rupert, and are, therefore, called by his name. They possess this singular property, that if a small portion of the tail is broken off, the whole bursts into powder with an explosion; and a considerable shock is communicated to the hand that grasps it.

The Bologna or philosophical phial, is a small vessel of glass, which has been suddenly cooled, open at the upper end, and rounded at the bottom. It is made so thick at the bottom, that it will bear a smart blow against a hard body, without breaking; but if a little pebble, or piece of flint, is let fall into it, it immediately cracks, and the bottom falls into pieces: but unless the pebble or flint is large and angular enough to scratch the surface of the glass, it will not break.

The most generally received explanation of these facts is founded on the assumption, that the dimensions of those bodies which are suddenly cooled, are larger than those which are more gradually cooled. The dimensions, therefore, of the smooth external surface of these glasses which are suddenly cooled, are supposed to be larger than is adapted to the accurate envelopement of the internal part, which is necessarily cooled in a more gradual man

ner; if, therefore, by a crack or scratch, a disjunction of the cohesion takes place, in the internal surface, the hidden action of the parts which remained in a state of tension, to recover that of perfect cohesion, is supposed to effect the destruction of the the mass.

VARNISHING.

By varnish is understood a clear limpid fluid, capable of hardening, without losing its transparency; used by painters, gilders, &c. to give a lustre to their works, and to preserve and defend them from the air and moisture.

A coat of varnish ought to possess the following properties: 1. It must exclude the action of the air: because wood and metals are varnished to defend them from decay and rust. 2. It must resist water; for otherwise the effect of the varnish could not be permanent. 3. It ought not to alter such colours as are intended to be preserved by this means. It is necessary, therefore, that a varnish should be easily extended or spread over the surface, without leaving pores or cavities, that it should not crack or scale, and that it should resist water.

Resins are the only bodies that possess these properties, consequently they must form the basis of every varnish. For this purpose they must be dissolved, as minutely divided as possible, and combined in such a manner, that the imperfections of those that might be disposed to scale, may be corrected by others.

Resins may be dissolved by three agents: 1. by fixed or fat oil; 2. by volatile, or essential oil; 3. by spirit of wine. Accordingly, we have three kinds of varnish: fat, or oily varnish; essential oil varnish; and spirit varnish.

These agents are of such a nature as either to dry up and become hard, or to evaporate and fly off, leaving the resin fixed behind.

Varnishes should be carefully kept from dust, and in very clean vessels: they should be laid as thin and even as possible with a large flat brush, taking care to lay the strokes all one way. A warm room is best for varnishing in, as cold chills the varnish, and prevents it from laying even.

Varnishes are polished with pumice-stone and tripoli. The pumice-stone must be reduced to a very fine powder, and put upon a piece of serge moistened with water; with this the varnished substance is to be rubbed equally and lightly. The tripoli must also be reduced to a fine powder, and put upon a clean woollen cloth, moistened with olive-oil, with which the polishing is to be performed. The varnish is then to be wiped with soft linen, and, when quite dry, cleaned with starch, or Spanish-white, and rubbed with the palm of the hand, or with a linen cloth.

Fat Oil Varnish.

Fixed or fat oil will not evaporate; nor will it become dry of itself. To make it dry,

it must be boiled with metallic calces or oxides. Litharge is generally used for this purpose. Oil so prepared is called drying-oil. To accelerate the drying of oil varnish, oil of turpentine is added.

Gum-copal, and amber, are the substances principally employed in oil varnishes; the copal being whitest, is used for varnishing light, the amber for dark colours.

It is best to dissolve them before mixing them with the oil; because, by this means, they are in

less danger of being scorched, and at the same time the varnish is more beautiful. They should be melted in an iron pot over the fire: they are in a proper state for receiving the oil when they give no resistance to the iron spatula, and when they run off from it drop by drop.

To make oil varnish, pour four, six, or eight ounces of drying-oil among sixteen ounces of melted copal, or amber, by little and little, constantly stirring the ingredients at the same time, · with the spatula. When the oil is well mixed with the copal or amber, take it off the fire; and when it is pretty cool, pour in sixteen ounces of the essence of Venice turpentine. After the varnish is made, it should be passed through a linen cloth.

Oil varnishes become thick by keeping; but when they are to be used, it is only necessary to pour in a little Venice turpentine, and to put them a little on the fire. Less turpentine is necessary in summer than in the winter: too much oil hinders the varnish from drying; but when too little is used, it cracks, and does not spread properly.

Black Varnish for Coaches and Iron-Work.

This varnish is composed of asphaltum, resin, and amber, melted separately, and afterwards mixed; the oil is then added, and afterwards the turpentine, as directed above. The usual proportions are, twelve ounces of amber, two of resin, two of asphaltum, six of oil, and twelve of turpentine.

A Varnish for rendering Silk Water and Air-tight.

To render the linseed-oil drying, boil it with two ounces of sugar of lead, and three ounces of

litharge, for every pint of oil, till the oil has dissolved them; then put a pound of bird-lime, and half a pint of the drying oil into a pot of iron or copper, holding about a gallon; and let it boil gently over a slow charcoal fire, till the bird-lime ceases to crackle; then pour upon it two pints and a half of drying-oil, and boil it for about an hour longer, stirring it often with an iron or wooden spatula. As the varnish in boiling swells much, the pot should be removed from the fire, and replaced when the varnish subsides. While it is boiling, it should be occasionally examined, in order to determine whether it has boiled enough. For this purpose, take some of it upon the blade of a large knife, and after rubbing the blade of another knife upon it, separate the knives; and when, on their separation, the varnish begins to form threads between the two knives, it has boiled enough, and should be removed from the fire. When it is almost cold, add about an equal quantity of spirits of turpentine; mix both well together, and let the mass rest till the next day; then having warmed it a little, strain and bottle it. If it is too thick, add spirits of turpentine. This varnish should be laid upon the stuff when perfectly dry, in a lukewarm state; a thin coat of it upon one side, and, about twelve hours after, two other coats should be laid on, one on each side; and in twenty-four hours the silk may be used.

Mr. Blanchard's Varnish for Air-balloons.

Dissolve elastic gum (Indian-rubber), cut small, in five times its weight of spirits of turpentine, by keeping them some days together; then boil one ounce of this solution in eight ounces of drying