merly used in the grinding and polishing, the use ofmachinery now assists the operation, and has led to adiminution in the cost of production, and consequently toincreased consumption.
Section D.— Bottle-Glass, Water Pipes , and Tubing,
The manufacture of bottles is of very considerableimportance, from the amount and value of its produce.In France alone, the trade is estimated at above 00millions of bottles annually; and their value at morethau half a million sterling. The earliest and one of themost important manufactories of this species of glasswas established in 1294, at Quinquengrone, in Normandy .
The bottles used for the effervescing wines, and forcontaining certain acids, require very great care in theirfabrication. In the former, it is necessary that the com-ponent parts should be thoroughly mixed, when the massis in a state of fusion ; and that the glass should be ofequal thickness throughout, that in every part the bottlemay be equally strong, and able to resist the pressureof the fixed air confined within.
The loss of bottles by bursting, in the champagnetrade, is stated to amount to from 20 to 30 per cent.; anda machine has been invented for testing their strength,which ought to be equal to bear the pressure of from 25to 35 atmospheres. The price of bottles for this purposeis generally nearly double that for ordinary purposes.
In bottles intended to contain acids, care should betaken to combine chemically the alkali and the lime, so asnot to incur the risk of their being acted upon by the acid,and subjected to decomposition.
Section E.— Glass for Chemical and PhilosophicalPurposes.
The glass to which this section refers requires peculiarqualities, according to the purpose to which it is to be applied.Hardness, evaporation of the salts by a long-continuedexposure to heat, but which cannot be effected withoutdeteriorating the colour, are the most essential qualitiesfor glass of this description ; and will bring it under theheads, partly of bottle-glass, and partly of flint-glass.
Several other products, useful in the dairy, the farm,the garden, &c., may be referred to this section.
Section F.— Flint-Glass, or Crystal, with or withoutLead .
The glass comprised in this section is derived from theremotest antiquity. Among the excavations which havebeen made iu Egypt , Greece , Italy , &c., are found frag-ments of blown-glass, moulded, pressed, cut, engraved,mosaic, reticulated, &c., and adorned with cameos^of thefinest workmanship. Venice and Bohemia followed inthe same branch, and were only surpassed by the substi-tution of a metallic oxide in the manufacture, and theproduction of a silicate of lead and potash, in lieu of thesilicate of potash and lime to which they still adhere.
The first manufactory of flint-glass in England wasestablished in the Savoy House, in the Strand, in 1552,and another in Crutched-Friars; but it was not till longafterwards that the great improvements which havebrought it to its present state of perfection, by the use ofminium, or oxide of lead, and closed pots in the furnace,were introduced.
In 1G35, Sir Robert Mansell obtained a patent formaking glass of this description, in consideration of hisundertaking to employ pit-coal, instead of wood, in his ifurnaces. He was also granted, on the same grounds, the jexclusive right of importing drinking-glasses of fine ;quality from Venice, which were not made in England >till half a century later.
The use of coal was soon found to affect the colour jof the metal: closed pots were then used for the purpose \of obviating the evil, but from their use arose another jobjection. The fusion did not take place in the closedpots so rapidly as when the metal was more immediatelyexposed to the action of the fire. A larger supply of ;alkali was required to assist the fusion, and the colour ;was still affected. This led to the adoption of a metallic jdux, and the oxide of lead was applied in as large quan- |tities as the fusion would bear without an alteration of.
colour. This new process not only remedied the eril,but has been found to produce glass of the purest colon*,most brilliant effect, and most perfect in quality of anyyet made.
This method of fabrication was not so much requiredon the Continent as in England, wood being principadyused as fuel in the furnaces. It was not till 1 784 t) atthe metallic oxide was introduced in a small factory at St. Cloud . It was afterwards removed to Mont Ceiiis, marAutun , which possessed the advantage of coal in t ieneighbourhood, and was known as the glass-works of t ieQueen, and continued its works till 1827.
Another manufactory of crystal, or silicate of leul,using wood fuel and open pots, was established in 17H),at St. Louis (Moselle). It has continued to carry on avery extensive business during the last thirty years, aridonly yields in importance to the glass-works of Baearat,which were purchased iu 1810 by M. D’Artigues, whoseworks at Vouesch, near Namur , had become, by tieTreaty of 1815, a part of the Belgian territory, and whopreferred to carry on his business in his own country.
The coloured glasses of Bohemia, of which Count Harrach , Count Schaffgotsch , and others, have exhibitedvery beautiful specimens, and their imitations in Ger many , France , and England, are comprised in this sec-tion; and likewise the millejiori style of work, adoptedfor making presse-papiers and other ornaments, and soldby hundreds of thousands, which, by the extent of thetrade, have become a very important branch of manu-facture. '
The description of glass referred to in this sectionsustains the high reputation which it has earned, and isably supported in the Exhibition by the products exhi-bited by the Osiers, the Pellatts, the Powells, the Richard-sons, and many other exhibitors. The American glasslikewise is comprised in this section. It is a silicate ofprotoxide of lead, and is remarkable for the purity of itscolour.
Section G,— Optical Glass, Flint or Crown.
The properties which are usually considered as consti-tuting excellence in glass for ordinary purposes may .easily be attained; but in glasses intended for opticalinstruments, and to be employed in the examination ofobjects so remote or so minute as to require the mostumleviating accuracy, the difficulty of obtaining themetal sufficiently free from the defects to which glass isincident, had hitherto baffled every attempt to produce alens, except of very small dimensions.
Purity, unchangeableness of colour, transparency, anda certain degree of refractive power may be obtained ;but perfect uniformity in the structure of the glass, so asto render its composition absolutely homogeneous iu allits parts, is not so easy to be accomplished; and it is pre-cisely this quality which is the most indispensable in themanufacture of optical glass.
The great difficulty seems to occur in the difference ofthe specific gravity of the several constituents of themetal: some melt at a lower temperature, and sinkingthrough the mixture, leave a streak or trail in descending ;some evaporate or decompose in a heat required for thefusion of others: and different substances cool at differenttemperatures. Hence arise discoloration, sweating, threads,globules, striae, irregular crystallization, which cause irre-gular refraction by the interruption of the rays of light,and their deflection from the course which they ought topursue.
The discovery of the achromatic telescope has been ofthe utmost importance in the science of astronomy. Thefirst idea is due to Galileo; but Euler , in recent times,applying his mind to the investigation of the cause offrequent failure in the construction of the telescope,adverted to the perfection of the arrangement of the crys-talline humours of the eye, bestowed by the bounty ofProvidence on man, which are so disposed as to providefor the variation in the different rays of light, and drawthem to one focus: and he imagined that glasses ofdifferent media might, on the same principle, be so»adapted to one another as to correct and regulate the dis-persive powers of the different rays. But the experiment