To one

selection, be rendered more perfect to the eye. simple test, that of their hardness, recourse can be had so easily, that every one may, with very little previous instruction, ascertain for himself the genuineness of any gem that is offered to his notice, without any apprehension of being deceived.

CHAP. VIII.

ON THE MANUFACTURE OF GLASS FROM CALCINED BONES.

PREPARATION OF BONES. -THEIR VITRIFICATION. PROCESS KNOWN TO BECHER. - CONCEALED BY HIM. CURIOUS SUGGESTION AS TO ITS EMPLOYMENT. THIS GLASS HIGHLY ELECTRIC WHEN NEWLY MADE.

GLASS may be made from calcined bones by digesting them during two or three days with half their weight of sulphuric acid, evaporating to dryness, and washing the residue in many different waters, until all the soluble matter is exhausted. The production of this effect is known by the water having no longer a yellow tinge.

The different waters thus used must then be brought together and evaporated to afford a solid extract. Το separate the sulphate of lime contained in this, the extract must be dissolved in the least possible quantity of water, and filtered: the salt will then remain on the filter. This extract may be mixed with powdered charcoal, and distilled for the production of phosphorus; but if, instead of this, it be placed in a large crucible, and the fire is urged, it will at first swell considerably, but ere long will again settle, and at that instant the glass is made. This is white, and of a milky colour.

These directions are taken from the System of Chemistry of M. Chaptal; who tells us that before his time Becher was perfectly well acquainted with the use to which bones could thus be applied, but that he concealed the process, on account of the abuse, which, according to his apprehensions, might be made of it, and to which he plainly enough alludes in the words" Homo vitrum est, et in vitrum redigi potest; sicut et omnia animalia." This author was, nevertheless, led to express his regret that the Scythians, who drank from disgusting skulls, were not acquainted with the art of converting them into so cleanly a substance as glass; and he also showed the possibility of forming a gallery of family

effigies, moulded from glass, the produce of the identical bones of the originals, in which the likenesses might be preserved as truly as they now are by the limner. M. Chaptal adds, that a skeleton of nineteen pounds' weight may be made to yield five pounds of this phosphoric glass.

Newly made glass of this description will emit very strong electric sparks, which will fly to the hand at the distance of two inches; but this property ceases after one or two days, however carefully the glass may be preserved from contact with the atmosphere. The substance is in fact phosphoric acid which has been deprived of its water, and which if not carefully preserved from the atmosphere it will again imbibe, becoming deliquescent. It has an acid taste, and is soluble in water.

CHAP. IX.

ON THE USE MADE OF THE BLOWPIPE, AND ON VARIOUS SMALL MANUFACTURES OF GLASS.

THERMOMETER TUBES. MODE OF GIVING TO THEM AN ELLIPTICAL BORE.- BLOWPIPE AND APPARATUS DESCRIBED. - MATERIALS USED. METHOD OF WORKING. SEALING TUBES. BENDING AND JOINING TUBES. BULBS. SPUN GLASS. WATCH-GLASSES. LUNETTE GLASSES. GLASS BEADS. — MANUFACTORY AT MURANO. STRIPED TUBES. MODE OF FORMING BEADS. SORTING THEM. NUMEROUS KINDS OF Beads. -MOCK PEARLS. MANNER OF THEIR INVENTION AND FORMATION. DIAL PLATES. HOW FORMED. LETTERING

AND FIGURING.

A CONSIDERABLE number of articles are made of glass with the aid of a lamp and blowpipe. The principal of these articles, such as thermometers and barometers, are formed from tubes which are made at the glass-houses, of different bores and substances, by drawing out quickly, and while soft with heat, a thick and short tube into one that is thin and long.

Fig. 34.

The method of performing this process is, to gather the necessary weight of glass upon the rod; and this glass having been elongated and hollowed by the workman's breath in the usual manner, a punt is attached to the end of the cylinder opposite to the rod. The workman then holding the rod and his assistant the punt, each proceeds in a direction opposite to that taken by the other, by which means the tube is elongated in the necessary degree; and being then made to rest upon billets of wood placed horizontally and parallel to each other at equal distances on the ground, it speedily cools, and may in that state be readily cut into convenient lengths.

Fig. 35.

Whatever may be the original form of this tube and of its perforation, it is found that the same form will be relatively continued throughout the entire length to which it is drawn out. If its perforation be at first cylindrical, it will so continue, whatever degree of length and minuteness it may be made to assume; and an equal sameness will be found to accompany the prolongation, any other form be originally given to the orifice.

if

A method has been suggested by Mr. Wilson, of Glasgow, to render the mercury in a thermometer tube easily observable, without incurring the inconveniencies which attend a large bore. This method is founded on the property above mentioned, and which is indeed common to all ductile substances.* Mr. Wilson proposes to form the tubes with an elliptical perforation, which when drawn out will form a mere slit, the flat side of which is to be turned towards the observer. It does not appear,

*For a beautiful scientific application of this principle, see Lardner's Mechanics, Cab. Cyc. p. 9. art. (12.)