THE NEW-YORK EXHIBITION ILLUSTRATED.

spirally colored drinking vessels, the letter weights with interior clusters offlowers and other colored ornaments, beads, aventurine, &c.

We will briefly describe some of these processes as we have seen thempractised in the ancient glass houses of Murano. Nowhere is the art of pro-ducing numerous and brilliant tints of colored glass better understood than inVenice. The pot metal is employed is the flint glass without lead, although leadis used to render some of the colored enamels more fusible. To take the simplestcase, that of a drinking glass whose tall stem involves a graceful spiral of severalthreads of white enamel in colorless glass. Cylindrical rods of glass about thesize of a pen stalk are drawn, and of any convenient length; these are colorless,and also of every tint of color which can bo named, transparent, opaque or opaline,as the case may be. A mass of colorless pot metal is taken from the furnace, andfashioned on the marver into a cylindrical form; while this is being reheated, ano-ther workman has broken several white enamel rods to the same length as theglass cylinder, and has also heated them to the softening point in the mouth ofthe furnace. The first workman now brings his heated cylinder of colorlessglass, parallel to the enamel sticks, and one by one attaches them to his cylinder bysimple contact, accurately dividing the space by his eye so that the enamelsticks are equally distant from each other. lie now rolls the compound and stillsoft mass upon the marver until the white cylinders are incorporated into the sub-stance of the colorless glass, but the relative distances are still accurately pre-served. Another assistant with a small mass of hot glass on the end of his puntingrod now approaches and fastens it to the fore end of the cylinder of glass still hotenough to yield to pressure; and as soon as the attachment is made the two work-men twist their rods in opposite directions, w'hich has the effect to give a specialtwist to the glass cylinder and its attached filaments of white enamel. This pro-cess is continued until the spiral is judged to be sufficiently close, when the massis again heated, and drawn out by the ordinary process of drawing glass rods,until it has acquired the desired size. A section from this spiral rod forms thestem of a wine glass, or several bits of equal length, placed side by side andreheated, may be made the means of a new and more complex spiral column bya repetition of the process just described. In this manner rods are found of vari-ously colored spiral threads most tastefully intertwined, every color that can be nam-ed being in turn selected and heated in the same manner, alone, or in combinationwith others. The spirals are now from left to right, and again the reverse, andboth are often seen in the same stem or rod. Parallel threads of color -are pro-duced with more or less ease. Conceive, then, all the prismatic colors, transparent,opaque, or opaline, combined in an almost endless series of such rods as have beendescribed, and placed at the command of an adroit workman—what wonderscan he not produce by their skilful combination ? Placed side by side upon aplate of iron in the heat of the furnace, such a series of rods can be broughtto the softening point, when they will adhere like so many sticks of sugar candyin warm weather. When they are in this condition a workman approaches witha disc of hot glass upon the end of his rod of such diameter as will measure in onerevolution exactly 7 , the breadth occupied by the softening spiral rods. He gentlyrolls the edge of his disc over the hither extremity of the soft rods, which are imme-diately gathered by it into a fluted open cylinder. This he further softens at thefurnace, and by rolling it on the marver he gathers in the open end until he closesit entirely, then applying himself to inflation he blows whatever form of vessel hewill from it, fashioning it by his turning tool and scissors at his pleasure. Thusin much less time than it has required to describe liis steps, we have a curious en-twined and various colored vessel of oriental grace, a perfect miracle of com-plexity when we recall the simple elements comprising it.

It is easy to understand that out of the same pliant and parti-colored rod,those ornaments of infinite variety may be formed, whose presence in letterweights has puzzled so many. It is only requisite to a better understanding ofthis curious product to remember that the white glass forming the transparentmass of the ornament is composed of much more fusible materials than thecolored central florets. The latter are fashioned at the blow-pipe table, out of thevery spiral and colored rods whose origin has been already described; and be-fore they are inclosed in their crystaline refracting mausoleum they have nospecial beauty. A mass of soft glass sufficient for the lower half of such a letterweight is now prepared, and upon its hot surface the colored floret or ornamentis applied, while immediately another workman approaches with a second hemis-pherical mass of colorless glass which he applies upon the upper surface of theornament. Thus one compound mass is produced having the ornamental glass inits centre, and after being duly fashioned, and annealed, and cut, forms the wonderwhich we see.

“We have already described the mode in which the surface of the Bohemiancrystal is flashed over with a film of ruby 7 or other colored glass. It will be readilyunderstood that the cutting away of a part of the colored surface will leave the 1colorless ground in bold contrast. The engraving of glass is a distinct artand requires the same kind of skill as that requisite for the production of cameosand intaglios, which was so well understood by the ancients. Very good diagramsof the processes of glass grinding and engraving will be found in Knapp’s AppliedChemistry, Yol. II., article Glass, from which we have made large drafts already.

Among the curious things of ancient Egyptian art in the collection of Dr.Abbott, so long on view in New-York, was a glass ornament with a chromaticinterior floret resembling so nearly the Venetian letter weight of modern timesas to leave no doubt (granting the genuineness of the object) that all the processesof the modern glass house were then in use.

Glass beads have been made from very ancient times in Venice where the art isstill practised. It will readily be understood that the variously colored rods alreadydescribed may be as easily formed tubular as solid. One of the peculiarities of glassis, when heated, to round itself on the sharp edges. When beads are to be formed,colored tubes of glass drawn down to the proper diameter are cut up into pieces ofthe proper length, and a large number of these are cautiously heated, when theiredges contract and become rounded into the form of beads. This operation is per-formed in a revolving cylinder of iron, in which the glass fragments are tumbledabout by the revolutions of the cylinder, mingled with dry lime and charcoal, to pre-vent them from agglomerating when softened. A tine collection of the Venetianbeads, mosaic glass enamels ( millefiori ) and aventurine, may be seen in the Austriancourt. The Venetian Aventurine owes its spangles of gold color to the presenceof small particles of snb-oxyd of copper (or as some chemists say 7 , of metallic cop-per), in an opaque ground. Among the modern uses of glass which are most pro-mising of future usefulness are the adaptation of large and strong pipes or tubesof glass for the conveyance of water and other fluids; and also casting of roughplates of strong cheap glass for roofs and floors of buildings. As glass is, inreality, one of the cheapest of manufactured products, and also one of the strongest,when formed of the more common materials, and when used for conveying fluidsits color is a matter of no moment, it is easy to believe it may easily take the placeof lead in conveying water, and thus avoid all the risk of injury to health whichis confessedly inseparable from the use of that metal.

Optical Glass .—The demands of physical science have not been easily met bythe glass maker, who, until a very recent period, has been unable to supply withany certainty even moderately large masses of faultless glass. To be* faultless foroptical purposes, glass must have a uniform density, a high refracting power (ifflint glass) colorlessness, freedom from striae, and lastly, an absence of air bubbles.To meet these requirements has staggered the resources of the whole scientificworld, who have by the most able commissions investigated this subject with thegreatest care both in England, France and Germany. For some time Frauenhoferwas believed to be the only person who, by a process Becret with himself, conldmake large lenses for refracting telescopes free from strioa and other imperfections.This was early in the present century, and long before Faraday had made hiscelebrated researches as head of the Commission of the Royal Society for inves-tigating the subject. We will not repeat the history of this interesting subject,which has been so often discussed, and may be found in all the standard works.Suffice it to say, the difficulty has been overcome, and glass discs of any requireddimensions may now be made with considerable degree of certainty that theywill be free from serious imperfections. The difficulties which so long stood inthe way of perfecting this branch of the glass maker’s art, were chiefly, theexistence of strim from inequalities of density in different parts of the mass, thepresence of air bubbles, which were given off in a late stage of the process of fusion,and the deterioration of color from the implements and means employed in stirring.Guinard, a pupil of Frauenhofer, introduced the practice of stirring the moltenmass in the pot by means of a stirrer composed of the same materials as the potitself, in place of an iron rod before used. This simple expedient, combined withgreat skill, especially in the construction of his furnace, and in the process of an-nealing, has enabled M. Bontemps to produce and exhibit in London in 1851, adisc of faultless flint glass of 29 inches diameter, and weighing over 200 pounds.The jury of Class V. have in the Jury Reports rendered a most interestingaccount of this remarkable flint glass disc, which was ground and finished in sucha manner that it could be submitted to all the most searching optical tests, notomitting the use of polarized light. When we remember that thejoint efforts ofFrauenhofer and Utzschneider of Munich produced only lenses of 9 inches diameter,and that in 1828 M. Bontemps was regarded as having produced a true marvel ofoptical art in turning out a lens of IT inches diameter, it will readily be under-stood that the late achievements of the same gentleman in the well known estab-lishment of Messrs. Glance, Brothers & Co., in Birmingham (where he is now per-manently connected), should have received the unqualified approval of such menas Sir David Brewster, Sir John Ilerschel, Lord Wrotlieslej 7 , Prof. Miller, Mr. Simms,and Mr. Ross. The density of this mass was 3.50 to 3.58, and its thicknessabout 21 inches. The composition of Bontemps’ flint glass is 200 lhs. of puresand, as much pure minium, and 60 lbs. of calcined soda. The metal is stirredduring thirty-three hours, and until the stirrer is moved with difficulty. Thefurnace is then closed, and suffered to cool for about eight days, when the coldmass of glass is broken out of the pot and its opposite faces ground to determineits quality. Subsequently it is cut up into discs of such size as may be required,which are then softened by heat and pressed in a mould into the rough form ofthe future lens. This can be accomplished without injury to the glass. M.Bontemps offered, some time since, to the French Institute, through M. Arago,to furnish lenses for a telescope 22 inches in diameter, at the following rates :—