See also:

• GLASS
• GLASS (O.E. glees, cf. Ger. Glas, perhaps derived from an old Teutonic root gla-, a variant of glo-, having the general sense of shining, cf. " glare," " glow ")
• GLASS, STAINED

GLASS (O.E. glees, cf. Ger. See also:

• GLAS, GEORGE (1725-1765)
• GLAS, JOHN (1695-1773)

Glas, perhaps derived from an old See also:

• TEUTONIC
• TEUTONIC (GERMANIC) LANGUAGES

Teutonic See also:

• ROOT (late O.E. rot, adopted from Scand., cf. Norw. and Swed. rot, Dan. rod; the true O.E. word was wyrt, plant, represented in Ger. Wurz or Wurzel; the ultimate root is the same in both words, and is seen in Lat. radix)
• ROOT, ELIHU (1845– )

root gla-, a variant of glo-, having the See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general sense of shining, cf. " glare," " glow ")  , a hard substance, usually trans-See also:

• PARENT, SIMON NAPOLEON (1855– )

parent or translucent, which from a fluid See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition at a high temperature has passed to a solid condition with sufficient rapidity to prevent the formation of visible crystals . There 'The name.See also:

• GLASITES, or SANDEMANIANS

Glasites or Glassites was generally used in See also:

• SCOTLAND
• SCOTLAND, CHURCH OF
• SCOTLAND, EPISCOPAL CHURCH OF

Scotland ; in See also:

• ENGLAND
• ENGLAND, THE CHURCH OF

England and See also:

• AMERICA

America the name Sandemanians was more See also:

• COMMON

common . are many varieties of See also:

• GLASS
• GLASS (O.E. glees, cf. Ger. Glas, perhaps derived from an old Teutonic root gla-, a variant of glo-, having the general sense of shining, cf. " glare," " glow ")
• GLASS, STAINED

glass differing widely in chemical See also:

• COMPOSITION
• COMPOSITION (Lat. compositio, from componere, to put together)

composition and in See also:

• PHYSICAL

physical qualities . Most varieties, however, have certain qualities in common . They pass through a viscous See also:

• STAGE (Fr. 6/age; from Lat. stare, to stand)

stage in cooling from a See also:

• STATE
• STATE, GREAT OFFICERS OF

state of fluidity; they develop effects of See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour when the glass mixtures are fused with certain metallic oxides; they are, when See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

cold, See also:

• BAD, BCD

bad conductors both of See also:

• ELECTRICITY

electricity and See also:

• HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat, they are easily fractured by a See also:

• BLOW, JOHN (1648-1708)

blow or See also:

• SHOCK, or COLLAPSE

shock and show a conchoidal fracture; they are but slightly affected by See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary solvents, but are readily attacked by hydrofluoric See also:

• ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid . The structure of glass has been the subject of repeated investigations . The theory most widely accepted at See also:

• PRESENT

present is that glass is a quickly solidified See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

solution, in which See also:

• SILICA

silica, silicates, borates, See also:

• PHOSPHATES

phosphates and aluminates may be either solvents or solutes, and metallic oxides and metals may be held either in solution or in suspension . See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

Long experience has fixed the mixtures, so far as ordinary See also:

• FURNACE

furnace temperatures are concerned, which produce the varieties of glass in common use . The essential materials of which these mixtures are made are, for See also:

• ENGLISH

English See also:

• FLINT
• FLINT, AUSTIN (1812-1886)
• FLINT, or FLTNTSHTRE (sir Gallestr)
• FLINT, ROBERT (1838- )
• FLINT, TIMOTHY (1780-1840)

flint glass, See also:

• SAND
• SAND, GEORGE (1804-1876)

sand, carbonate of potash and red See also:

• LEAD
• LEAD (pronounced iced)

lead; for See also:

• PLATE

plate and See also:

• SHEET

sheet glass, sand, carbonate or sulphate of soda and carbonate of See also:

• LIME
• LIME (O. Eng. lim, Lat. limes, mud, from linere, to smear)

lime; and for Bohemian glass, sand, carbonate of potash and carbonate of lime . It is convenient to treat these glasses as " normal " glasses, but they are in reality mixtures of silicates, and cannot rightly be regarded as definite chemical compounds or represented by definite chemical formulae . The knowledge of the See also:

• CHEMISTRY
• CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see ALCHEMY)

chemistry of glass-making has been considerably widened by Dr F . O .

Schott's experiments at the See also:

• JENA

Jena glass-See also:

• WORKS

works . The commercial success of these works has demonstrated the value of pure See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

science to manufactures . The See also:

• RECENT

recent large increase in the number of varieties of glass has been chiefly due to developments in the manufacture of See also:

• OPTICAL

optical glass . Glasses possessing See also:

• SPECIAL

special qualities have been required, and have been supplied by the introduction of new combinations of materials . The range of the specific gravity of glasses from 2•5 to 5•o illustrates the effect of modified compositions . In the same way glass can be rendered more or less fusible, and its stability can be increased both in relation to extremes of temperature and to the chemical See also:

• ACTION

action of solvents . The fluidity of glass at a high temperature renders possible the processes of ladelling, pouring, casting and stirring . A See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass of glass in a viscous state can be rolled with an See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron See also:

• ROLLER

roller like dough; can be rendered hollow by the pressure of the human breath or by compressed See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air; can be forced by air pressure, or by a mechanically driven plunger, to take the shape and impression of a See also:

• MOULD

mould; and can be almost indefinitely extended as solid See also:

• ROD (O.E. rodd, probably related to Norw. rudda, stick, rodda, stake)
• ROD, EDOUARD (1857-1910)

rod or as hollow See also:

• TUBE (Lat. tuba)

tube . So extensible is viscous glass that it can be See also:

• DRAWN

drawn out into a filament sufficiently See also:

• FINE

fine and elastic to be See also:

• WOVEN

woven into a fabric . Glasses are generally transparent but may be translucent or opaque . Semi-opacity due to See also:

• CRYSTALLIZATION

crystallization may be induced in many glasses by maintaining them for a long See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period at a temperature just insufficient to cause See also:

• FUSION

fusion . In this way is produced the crystalline, devitrified material, known as See also:

• REAUMUR, RENE ANTOINE FERCHAULT DE (1683-1757)

Reaumur's See also:

• PORCELAIN

porcelain .

Semi-opacity and opacity are usually produced by the addition to the glass-mixtures of materials which will remain in suspension in the glass, such as See also:

• OXIDE

oxide of See also:

• TIN (Lat.- stannum, whence the chemical symbol " Sn "; atomic weight =117.6, 0=16)

tin, oxide of See also:

• ARSENIC (symbol As, atomic weight 75.0)

arsenic, phosphate of lime, See also:

• CRYOLITE

cryolite or a mixture of See also:

• FELSPAR, or FELDSPAR

felspar and fluorspar . Little is known about the actual cause of colour in glass beyond the fact that certain materials added to and melted with certain glass-mixtures will in favourable circumstances produce effects of colour . The colouring agents are generally 'metallic oxides . The same oxide may produce different See also:

• COLOURS, MILITARY

colours with different glass-mixtures, and different oxides of the same See also:

• METAL
• METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. µATaXAov, in the same sense, probably connected with ,ueraAAdv, to search after, explore, µeTa, after, aAAos, other)

metal may produce different colours . The See also:

• PURPLE

purple-See also:

• BLUE (common in different forms to most European languages)

blue of See also:

• COBALT (symbol Co, atomic weight 59)

cobalt, the chrome See also:

• GREEN, A
• GREEN, ALEXANDER HENRY (1832—1896)
• GREEN, DUFF (1791—1875)
• GREEN, JOHN RICHARD (1837—1883)
• GREEN, MATTHEW (1696-1737)
• GREEN, THOMAS HILL (1836-1882)
• GREEN, VALENTINE (1739–1813)
• GREEN, WILLIAM HENRY (.1825–1900)

green or yellow of See also:

• CHROMIUM (symbol Cr. atomic weight 52.1)

chromium, the dichroic See also:

• CANARY
• CANARY (Serinus canarius)

canary-colour of See also:

• URANIUM [symbol U, atomic weight 238.5 (0=16)]

uranium and the See also:

• VIOLET

violet of See also:

• MANGANESE (symbol Mn; atomic weight, 54.93 (0=16)], a metallic chemical element. Its dioxide (pyrolusite)

manganese, are See also:

• CONSTANT, BENJAMIN (1845-1902)

constant . Ferrous oxide produces an See also:

• OLIVE (Olea europaea)

olive green or a See also:

• PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

pale blue according to the glass with which it is mixed . Ferric oxide gives a yellow colour, but requires the presence of an oxidizing See also:

• AGENT (from Lat. agere, to act)

agent to prevent reduction to the ferrous state . Lead gives a pale yellow colour . See also:

• SILVER

Silver oxide, mixed as a paint and spread on the See also:

• SURFACE

surface of a piece of glass and heated, gives a permanent yellow stain . Finely divided See also:

• VEGETABLE

vegetable See also:

• CHARCOAL

charcoal added to a soda-lime glass gives a yellow colour . It has been suggested that the colour is due to See also:

• SULPHUR

sulphur, but the effect can be produced with a glass mixture containing no sulphur, See also:

• FREE

free or combined, and by increasing the proportion of charcoal the intensity of the colour can be increased until it reaches See also:

• BLACK
• BLACK, ADAM (1784-1874)
• BLACK, JEREMIAH SULLIVAN (1810–1883)
• BLACK, WILLIAM (1841-1898)

black opacity . Selenites and selenates give a pale See also:

• PINK

pink or pinkish yellow .

See also:

• TELLURIUM [Symbol Te, atomic weight 127.5 (0=16)]

Tellurium appears to give a pale pink tint . See also:

• NICKEL
• NICKEL (symbol Ni, atomic weight 58.68 (0=16))

Nickel with a potash-lead glass gives a violet colour, and a See also:

• BROWN
• BROWN, CHARLES BROCKDEN (1771-181o)
• BROWN, FORD MADOX (1821-1893)
• BROWN, FRANCIS (1849- )
• BROWN, GEORGE (1818-188o)
• BROWN, HENRY KIRKE (1814-1886)
• BROWN, JACOB (1775–1828)
• BROWN, JOHN (1715–1766)
• BROWN, JOHN (1722-1787)
• BROWN, JOHN (1735–1788)
• BROWN, JOHN (1784–1858)
• BROWN, JOHN (1800-1859)
• BROWN, JOHN (1810—1882)
• BROWN, JOHN GEORGE (1831— )
• BROWN, ROBERT (1773-1858)
• BROWN, SAMUEL MORISON (1817—1856)
• BROWN, SIR GEORGE (1790-1865)
• BROWN, SIR JOHN (1816-1896)
• BROWN, SIR WILLIAM, BART
• BROWN, THOMAS (1663-1704)
• BROWN, THOMAS (1778-1820)
• BROWN, THOMAS EDWARD (1830-1897)
• BROWN, WILLIAM LAURENCE (1755–1830)

brown colour with a soda-lime glass . See also:

• COPPER (symbol Cu, atomic weight 63.1, H=1, or 63.6, O = 16)

Copper gives a See also:

• PEACOCK (Lat. Pavo, O. Eng. Pawe, Du. pauuw, Ger. Pfau, Fr. Paon)
• PEACOCK, GEORGE (1791–1858)
• PEACOCK, SIR BARNES (1810-189o)
• PEACOCK, THOMAS LOVE (1785-1866)

peacock-blue which becomes green if the See also:

• PRO

pro-portion of the copper oxide is increased . If oxide of copper is added to a glass mixture containing a strong reducing agent, a glass is produced which when first taken from the crucible is colourless but on being re-heated develops a deep See also:

• CRIMSON

crimson - See also:

• RUBY (Lat. rubeus, red)

ruby colour . A similar glass, if its cooling is greatly retarded, produces throughout its substance See also:

• MINUTE
• MINUTE (Lat. minutes, small; minuere, to make less)

minute crystals of metallic copper, and closely resembles the See also:

• MINERAL

mineral called avanturine . There is also an intermediate stage in which the glass has a rusty red colour by reflected See also:

• LIGHT

light, and a purple-blue colour by transmitted light . Glass containing See also:

• GOLD
• GOLD [symbol Au, atomic weight 195.7(H = 1),197.2(0 =16)]

gold behaves in almost precisely the same way, but the ruby glass is less crimson than copper ruby glass . J . E . C . See also:

• MAXWELL
• MAXWELL, JAMES CLERK (1831–1879)

Maxwell See also:

• GARNETT, RICHARD (1835–1906)

Garnett, who has studied the optical properties of these glasses, has suggested that the changes in colour correspond with changes effected in the structure of the metals as they pass gradually from solution in the glass to a state of crystallization . Owing to impurities contained in the materials from which glasses are made, accidental coloration or discoloration is often produced .

For this See also:

• REASON (Lat. ratio, through French raison)

reason chemical agents are added to glass mixtures to remove or neutralize accidental colour . Ferrous oxide is the usual cause of discoloration . By converting ferrous into ferric oxide the green tint is changed to yellow, which is less noticeable . Oxidation may be effected by the addition to the glass mixture of a substance which gives up See also:

• OXYGEN (symbol 0, atomic weight 16)

oxygen at a high temperature, such as manganese dioxide or arsenic trioxide . With the same See also:

• OBJECT

object, red lead and See also:

• SALTPETRE (from the Lat. sal, salt, Petra, a rock)

saltpetre are used in the mixture for potash-lead glass . Manganese dioxide not only acts as a source of oxygen, but develops a pink tint in the glass, which is complementary to and neutralizes the green colour due to ferrous oxide . Glass is a bad conductor of heat . When boiling See also:

• WATER

water is poured into a glass See also:

• VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel, the vessel frequently breaks, on See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account of the unequal expansion of the inner and See also:

• OUTER

outer layers . If in the See also:

• PROCESS

process of glass manufacture a glass vessel is suddenly cooled, the constituent particles are unable to arrange themselves and the vessel remains in a state of extreme tension . The surface of the vessel may be hard, but the vessel is liable to fracture on receiving a trifling shock . M. de la Bastie's process of " toughening " glass consisted in dipping- glass, raised to a temperature slightly below the melting-point, into molten See also:

• TALLOW (M.E. talugh, talg, cf. Du. talk, L. Ger. talg ; the connexion with O.E. taelg, dye, or Goth, tulgus, firm, is doubtful)

tallow . The surface of the glass was hardened, but the inner layers remained in unstable See also:

• EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium .

Directly the crust was pierced the whole mass was shattered into minute fragments . In all branches of glass manufacture the process of " See also:

• ANNEALING, HARDENING AND

annealing," i.e. cooling the manufactured See also:

• OBJECTS

objects sufficiently slowly to allow the constituent particles to See also:

• SETTLE
• SETTLE, ELKANAH (1648–1724)

settle into a condition of equilibrium, is of vital ' importance . The desired result is obtained either by moving the manufactured goods gradually away from a constant87 source of heat, or by placing them in a heated See also:

• KILN (0. E. cylene, from the Lat. culina, a kitchen, cooking-stove)

kiln and allowing the heat gradually to See also:

• DIE
• DIE (Fr. de, from Lat. datum, given)

die out . The furnaces (fig . 15) employed for melting glass are usually heated with See also:

• GAS

gas on the " See also:

• SIEMENS, ERNST WERNER VON (1816-1892)
• SIEMENS, SIR WILLIAM [KARL WILHELM] (1823-1883)

Siemens," or some similar See also:

• SYSTEM

system of regenerative See also:

• HEATING

heating . In the See also:

• UNITED

United States natural gas is used wherever it is available . In some English works See also:

• COAL

coal is still employed for See also:

• DIRECT

direct heating with various forms of See also:

• MECHANICAL

mechanical stokers . Crude See also:

• PETROLEUM (Lat. Petra, rock, and oleuin, oil)

petroleum and a thin See also:

• TAR

tar, resulting from the process of enriching water-gas with petroleum, have been used A . Table glass . B . Tube . C .

She e t D . Bottles . Special glasses and See also:

• CROWN

crown for t he r m o- glass . meters, and other special glasses . A . Plate and rolled plate glass . B . Pressed table glass . I . OPTICAL GLASS.—As regards both mode of See also:

• PRODUCTION (Lat. productionem, from producere, to pro-duce)

production and essential properties optical glass differs widely from all other varieties . These See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences arise primarily from the fact that glass for optical uses is required in comparatively large and thick pieces, while for most other purposes glass is used in the See also:

• FORM (Lat. forma)

form of comparatively thin sheets; when, therefore, as a consequence n uaevar.rr.a.e.a.z.o.a.a ~.ara a~ .~~mra...a.ee=r\w"~ vim iNYd.'~v/LS nSOA~/Giol~ IDRt'" . ...~JOO.bi\ n .

both with compressed air and with See also:

• STEAM
• STEAM (0. Eng. steam, vapour, smoke, cf. Du. steam; the origin is unknown)

steam with considerable success . See also:

• ELECTRICAL
• ELECTRICAL (or ELECTROSTATIC) MACHINE

Electrical furnaces have not as yet been employed for ordinary glass-making on a commercial See also:

• SCALE
• SCALE (1) A

scale, but the electrical See also:

• PLANTS

plants which have been erected for melting and moulding See also:

• QUARTZ

quartz suggest the possibility of electric heating being employed for the manufacture of glass . Many forms of apparatus have been tried for ascertaining the temperature of glass furnaces . It is usually essential that some parts of the apparatus shall be made to acquire a temperature identical with the temperature to be measured . Owing to the physical changes produced in the material exposed prolonged observations of temperature are impossible . In the Fery See also:

• RADIATION, THEORY OF

radiation See also:

• PYROMETER (Gr. iri p, fire, µErpov, a measure)

pyrometer this difficulty is obviated, as the See also:

• INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument may be placed at a considerable distance from the furnace . The radiation passing out from an opening in the furnace falls upon a See also:

• CONCAVE

concave See also:

• MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror in a See also:

• TELESCOPE

telescope and is focused upon a thermoelectric couple . The hotter the furnace the greater is the rise of temperature of the couple . The electromotive force thus generated is measured by a See also:

• GALVANOMETER

galvanometer, the scale of which is divided and figured so that the temperature may be directly read . (See See also:

• THERMOMETRY (Gr. Bepµos, warm; µerpov, a measure)

THERMOMETRY.) In dealing with the manufacture of glass it is convenient to See also:

• GROUP

group the various branches in the following manner: Manufactured Glass . I . Optical Glass II .

Blown Glass of See also:

• DOLLOND, JOHN (1706—1761)

Dollond's invention of achromatic telescope objectives in led to such important developments that similar See also:

• WORK

work was undertaken in See also:

• FRANCE
• FRANCE, ANATOLE (1844– )

France by the See also:

• FIRM

firm of Mantois, the successors of Feil, and somewhat later by See also:

• CHANCE (through the O. Fr. cheance, from the Late Lat. cadentia, things happening, from cadere, to fall out, happen; cf. " case ")

Chance in England . The manufacture of the new varieties of glass, originally known as " Jena " glasses, is now carried out extensively and with a considerable degree of commercial success in France, and also to a less extent in England, but none of the other makers of optical glass has as yet contributed to the progress of the See also:

• INDUSTRY (Lat. industria, from indu-, a form of the pre, position in, and either stare, to stand, or struere, to pile up)

industry to anything like the same extent as the Jena firm . The older optical glasses, now generally known as the " ordinary " crown and flint glasses, are all of the nature of pure silicates, the basic constituents being, in the See also:

• CASE
• CASE, JOHN (d. 1600)

case of crown glasses, lime and soda or lime and potash, or a mixture of both, and in the case of flint glasses, lead and either (or both) soda and potash . With the exception of the heavier flint (lead) glasses, these can be produced so as to be free both from noticeable colour and from such defects as bubbles, opaque inclusions or " striae," but extreme care in the choice of all the raw materials and in all the manipulations is required to ensure this result . Further, these glasses, when made from properly proportioned materials, possess a.very considerable degree of chemical stability, which is amply sufficient for most optical purposes . The newer glasses, on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand, contain a much wider variety of chemical constituents, the most important being the oxides of See also:

• BARIUM (symbol Ba, atomic weight 137'37 [0=16])

barium, See also:

• MAGNESIUM [symbol Mg, atomic weight 24.32 (0 = 16)]

magnesium, See also:

• ALUMINIUM (symbol Al; atomic weight 27.0)

aluminium and See also:

• ZINC

zinc, used either with or without the addition of the bases already named in reference to the older glasses, and—among acid bodies—boric anhydride (B203) which replaces the silica of the older glasses to a varying extent . It must be admitted that, by the aid of certain of these new constituents, glasses can be produced which, as regards purity of colour, freedom from defects and chemical stability are equal or even See also:

• SUPERIOR

superior to the best of the " ordinary " glasses, but it is a remarkable fact that when this is the case the optical properties of the new glass do not fall very widely outside the limits set by the older glasses . On the other hand, the more extreme the optical properties of these new glasses, i.e. the further they depart from the ratio of refractive See also:

• INDEX

index to dispersive See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power found in the older glasses, the greater the difficulty found in obtaining them of either sufficient purity or stability to be of See also:

• PRACTICAL

practical use . It is, in fact, admitted that some of the glasses, most useful optically, the dense barium crown glasses, which are so widely used in See also:

• MODERN

modern photographic lenses, cannot be produced entirely free either from noticeable colour or from numerous small bubbles, while the chemical nature of these glasses is so sensitive that considerable care is required to protect the surfaces of lenses made from them if serious tarnishing is to be avoided . In practice, however, it is not found that the presence either of a decidedly greenish-yellow colour or of numerous small bubbles interferes at all seriously with the successful use of the lenses for the See also:

• MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of purposes, so that it is preferable to See also:

• SACRIFICE (from Lat. sacrificium; sacer, holy, and facere, to make)

sacrifice the perfection of the glass in See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order to secure valuable optical properties . It is a further striking fact, not unconnected with those just enumerated, that the extreme range of optical properties covered even by the relatively large number of optical glasses now available is in reality very small . The refractive indices of all glasses at present available See also:

• LIE, JONAS LAURITZ EDEMIL (1833—1908)
• LIE, MARIUS SOPHUS (1842–1899)

lie between 1.46 and 1 •qo, whereas transparent minerals are known having refractive indices lying considerably outside these limits; at least one of these, fluorite (See also:

• CALCIUM [symbol Ca, atomic weight 40.0 (o= x6)]

calcium fluoride), is actually used by opticians in the construction of certain lenses, so that probably progress is to be looked for in a considerable widening of the limits of available optical materials; possibly such progress may lie in the direction of the artificial production of large mineral crystals .

The qualities required in optical glasses have already been partly referred to, but may now be summarized: 177, a demand first arose for optical glass, the industry was unable to furnish suitable material . Flint glass particularly, which appeared quite satisfactory when viewed in small pieces, was found to be so far from homogeneous as to be useless for See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

lens construction . The first step towards overcoming this vital defect in optical glass was taken by P . L . Guinand, towards the end of the 18th See also:

• CENTURY (from Lat. centuria, a division of a hundred men)

century, by introducing the process of stirring the molten glass by means of a See also:

• CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

cylinder of fireclay . Guinand was induced to migrate from his See also:

• HOME
• HOME, DANIEL DUNGLAS (1833-1886)
• HOME, EARLS OF

home in See also:

• SWITZERLAND

Switzerland to See also:

• BAVARIA (Ger. Bayern)

Bavaria, where he worked at the production of homogeneous flint glass, first with See also:

• JOSEPH
• JOSEPH, COMTE DE (1785-1870)
• JOSEPH, FATHER (FRANCOIS LECLERC DU TREMBLAY) (1577-1638)

Joseph von Utzschneider and then with J . See also:

• FRAUNHOFER, JOSEPH VON (1787-1826)

Fraunhofer; the latter ultimately attained considerable success and produced telescope disks up to 28 centimetres (11 in.) See also:

• DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter . Fraunhofer further initiated the See also:

• SPECIFICATION (from Med. Lat. specificatio, specificare, to enumerate or mention in detail)

specification of See also:

• REFRACTION
• REFRACTION (Lat. refringere, to break open or apart)

refraction and See also:

• DISPERSION
• DISPERSION (from Lat. dispergere, to scatter)

dispersion in terms of certain lines of the spectrum, and even attempted an investigation of the effect of chemical composition on the relative dispersion produced by glasses in different parts of the spectrum . Guinand's process was further See also:

• DEVELOPED

developed in France by Guinand's sons and subsequently by Bontemps and E . Feil . In 1848 Bontemps was obliged to leave France for See also:

• POLITICAL

political reasons and came to England, where he initiated the optical glass manufacture at Chance's glass works near See also:

• BIRMINGHAM

Birmingham, and this firm ultimately attained a considerable reputation in the production of optical glass, especially of large disks for telescope objectives . Efforts at improving optical glass had, however, not been confined to the descendants and successors of Guinand and Fraunhofer .

In 1824 the Royal Astronomical Society of See also:

• LONDON

London appointed a See also:

• COMMITTEE (from commute, an Anglo-Fr. past participle of commettre, Lat. committere, to entrust; the modern Fr. equivalent comite is derived from the Eng.)

committee on the subject, the experimental work being carried out by See also:

• FARADAY, MICHAEL (1791-1867)

Faraday . Faraday independently recognized the See also:

• NECESSITY (Lat. necessitas)

necessity for mechanical agitation of the molten glass in order to ensure homogeneity, and to facilitate his manipulations he worked with dense lead borate glasses which are very fusible, but have proved too unstable for ordinary optical purposes .. Later Mites of See also:

• CLICHY

Clichy (France) exhibited some " zinc crown " glass in small plates of optical quality at the London See also:

• EXHIBITION

Exhibition of 1851; and another See also:

• FRENCH
• FRENCH, DANIEL CHESTER (1850– )
• FRENCH, NICHOLAS (1604-1678)

French glass-maker, Lamy, produced a dense See also:

• THALLIUM

thallium glass in 1867 . In 1834 W . V . See also:

• HARCOURT
• HARCOURT, SIMON HARCOURT, 1ST VISCOUNT (c. 1661-1727)
• HARCOURT, SIR WILLIAM GEORGE GRANVILLE VENABLES VERNON (1827-1904)
• HARCOURT, WILLIAM VERNON (1789-1871)

Harcourt began experiments in glass-making, in which he was subsequently joined by G . G . See also:

• STOKES, SIR G
• STOKES, SIR GEORGE GABRIEL, BART
• STOKES, WHITLEY (1830-1909)

Stokes . Their object was to pursue the inquiry begun by Fraunhofer as to the effect of chemical composition on the See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

distribution of dispersion . The specific effect of boric acid in this respect was correctly ascertained by Stokes and Harcourt, but they mistook the effect of titanic acid . J . See also:

• HOPKINSON, FRANCIS (1737–1791)
• HOPKINSON, JOHN (1849-1898)

Hopkinson, working at Chance's glass works, subsequently made an See also:

• ATTEMPT (Lat. adtemptare, attentare, to try)

attempt to produce a See also:

• TITANIUM [symbol Ti, atomic weight 48.1 (0 = 16)]

titanium silicate glass, but nothing further resulted .

The next and most important forward step in the progress of optical glass manufacture was initiated by See also:

• ERNST, HEINRICH WILHELM (1814–1865)

Ernst See also:

• ABBE (Serbo-Croatian Rai))

Abbe and carried out jointly by him and O . Schott at Jena in See also:

• GERMANY
• GERMANY (Ger. Deutschland)

Germany . Aided by grants from the Prussian See also:

• GOVERNMENT
• GOVERNMENT (0. Fr. governement, mod. gouvernement, O. Fr. governer, mod. gouverner, from Lat. gubernare, to steer a ship, guide, rule; cf. Gr. xv(3epvav)

government, these workers systematically investigated the effect of introducing a large number of different chemical substances (oxides) into vitreous fluxes . As a result a whole See also:

• SERIES (a Latin word from serere, to join)

series of glasses of novel composition and optical properties were produced . A certain number of the most promising of these, from the purely optical point of view, had unfortunately to be abandoned for practical use owing to their chemical instability, and the problem of Fraunhofer, viz. the production of pairs of glasses of widely differing refraction and dispersion, but having a similar distribution of dispersion in the various regions of the spectrum, was not in the first instance solved . On the other hand, while in the older crown and flint glasses the relation between refraction and dispersion had been practically fixed, dispersion and refraction increasing regularly with the See also:

• DENSITY (Lat. densus, thick)

density of the glass, in some of the new glasses introduced by Abbe and Schott this relation is altered and a relatively See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

low refractive index is accompanied by a relatively high dispersion, while in. others a high refractive index is associated with low dispersive power . The initiative of Abbe and Schott, which was greatly aided by the resources for scientific investigation available at the Physikalische Reichsanstalt (Imperial Physical Laboratory), 1 . Transparency and Freedom from Colour.—These qualities can be readily judged by inspection of the glass in pieces of considerable thickness, and they may be quantitatively measured by means of the spectro-photometer . 2 . Homogeneity.—The optical desideratum is uniformity of refractive index and dispersive power throughout the mass of the glass . This is probably never completely attained, See also:

• VARIATIONS
• VARIATIONS, CALCULUS
• VARIATIONS, CALCULUS OF

variations in the See also:

• SIXTH

sixth significant figure of the refractive index being observed in different parts of single large blocks of the most perfect glass . While such minute and See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual variations are harmless for most optical purposes, sudden variations which generally take the form of striae or See also:

• VEINS

veins are fatal defects in all optical glass .

In their coarsest forms such striae are readily visible to the unaided See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye, but finer ones See also:

• ESCAPE (in mid. Eng. eschape or escape, from the O. Fr. eschapper, modern echapper, and escaper, low Lat. escapium, from ex, out of, and cappa, cape, cloak; cf. for the sense development the Gr. iichueoOat, literally to put off one's clothes, hence to sli

escape detection unless special means are taken for rendering them visible; such special means conveniently take the form of an apparatus for examining the glass in a See also:

• BEAM
• BEAM (from the O. Eng. beam, cf. Ger. Baum, a tree, to which sense may be referred the use of " beam " as meaning the rood or crucifix, and the survival in certain names of trees, as horn-beam)

beam of parallel light, when the striae scatter the light and appear as either dark or See also:

• BRIGHT, JOHN (1811-188g)
• BRIGHT, SIR CHARLES TILSTON

bright lines according to the position of the eye . Plate glass of the usual quality, which appears to be perfectly homogeneous when looked at in the ordinary way, is seen to be a mass of fine striae, when a considerable thickness is examined in parallel light . Plate glass is, nevertheless, consider-ably used for the cheaper forms of lenses, where the scattering of the light and loss of See also:

• DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

definition arising from these fine striae is not readily recognized . Bubbles and enclosures of opaque See also:

• MATTER

matter, although more readily observed, do not constitute such serious defects; their presence in a lens, to a moderate extent, does not interfere with its performance (see above) . 3 . Hardness and Chemical Stability.-These properties contribute to the durability of lenses, and are specially desirable in the outer members of lens combinations which are likely to be subjected to frequent handling or are exposed to the See also:

• WEATHER (O. Eng. weder; the word is common to Teutonic languages; cf. Du. weder, Dan. veir, Icel. ve8r, and Ger. Wetter and Gewitter, storm; the root is wa- to blow, from which is derived " wind ")

weather . As a See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general See also:

• RULE

rule, to which, however, there are important exceptions, both these qualities are found to a greater degree, the See also:

• LOWER

lower the refractive index of the glass . The chemical stability, i.e. the power of resisting the disintegrating effects of atmospheric moisture and carbonic acid, depends largely upon the quantity of alkalis contained in the glass and their proportion to the lead, lime or barium present, the stability being generally less the higher the proportion of See also:

• ALKALI

alkali . A high silica-content tends towards both hardness and chemical stability, and this can be further increased by the addition of small proportions of boric acid; in larger quantities, however, the latter constituent produces the opposite effect . 4 . See also:

• ABSENCE (Lat. absentia)

Absence of See also:

• INTERNAL

Internal See also:

• STRAIN (through O. Fr. straindre, estraindre, mod. i treindre, from Lat. stringere, to draw tight, related to stress, stretch, string, &c.)

Strain.-Internal strain in glass arises from the unequal contraction of the outer and inner portions of masses of glass during cooling . Processes of annealing, or very gradual cooling, are intended to relieve these strains, but such processes are only completely effective when the cooling, particularly through those ranges of temperature where the glass is just losing the last traces of plasticity, Is extremely gradual, a See also:

• RATE

rate measured in See also:

• HOURS, CANONICAL

hours per degree Centigrade being required .

The existence of internal strains in glass can be readily recognized by examination in polarized light, any signs of See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double refraction indicating the existence of strain . If the glass is very badly annealed, the lenses made from it may See also:

• FLY (formed on the root of the supposed original Tent. fleugan, to fly)

fly to pieces during or after manufacture, but apart from such extreme cases the optical effects of internal strain are not readily observed except in large optical apparatus . Very perfectly annealed optical glass is now, however, readily obtainable . 5 . Refraction and Dispersion.-The purely optical properties of refractions and dispersion, although of the greatest importance, cannot he dealt with in any detail here; for an account of the optical properties required in glasses for various forms of lenses see the articles LENS and See also:

• ABERRATION (Lat. ab, from or away, errare,, to wander)

ABERRATION: II . In Optical Systems . As typical of the range of modern optical glasses Table I. is given, which constituted the See also:

• LIST (O.E. lisle, a Teutonic word, cf. Dut. lust, Ger. Leiste, adapted in Ital. lista and Fr. lisle)
• LIST, FRIEDRICH (1789-1846)

list of optical glasses exhibited by Messrs Chance at the Optical See also:

• CONVENTION (Lat. conventio, an assembly or agreement, from convenire, to come together)
• CONVENTION, THE NATIONAL

Convention in London in 1905 . In this table n is therefractive index of the glass for See also:

• SODIUM [symbol Na, from Lat. natrium; atomic weight 23.00 (0=16)]

sodium light (the D See also:

• LINE

line of the See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar spectrum), while the letters C, F and G' refer to lines in the See also:

• HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen spectrum by which dispersion is now generally specified . The See also:

• SYMBOL (Gr. o-uµ(3oXov, a sign)

symbol p represents the inverse of the dispersive power, its value being (nD-1)/(C-F) . The very much longer lists of See also:

• GERMAN
• GERMAN, DUTCH AND SCANDINAVIAN

German and French firms contain only a few types not represented in this table . Manufacture of Optical Glass.-In its earlier stages, the process for the production of optical glass closely resembles that used in the production of any other glass of the highest quality . The raw materials are selected with See also:

• GREAT

great care to assure chemical purity, but whereas in most glasses the only impurities to be dreaded are those that are either infusible or produce a colouring effect upon the glass, for optical purposes the admixture of other glass-forming bodies than those which are intended to be present must be avoided on account of their effect in modifying the optical constants of the glass .

Constancy of composition of the raw materials and their careful and thorough admixture in constant proportions are therefore essential to the production of the required glasses . The materials are generally used in the form either of oxides (lead, zinc, silica, &c.) or of salts readily decomposed by heat, such as the nitrates or See also:

• CARBONATES

carbonates . Fragments of glass of the same composition as that aimed at are generally incorporated to a limited extent with the mixed raw materials to facilitate their fusion . The crucibles or pots used for the production of optical glass very closely resemble those used in the manufacture of flint glass for other purposes; they are " covered " and the molten materials are thus protected from the action of the furnace gases by the interposition of a See also:

• WALL (O. Eng. weal, weall, Mid. Eng. wal, wane, adapted from Lat. vallum, rampart; the original O. Eng. word for a wall was wag or tenth)
• WALL, RICHARD (1694-1778)

wall of fireclay, but as crucibles for optical glass are used for only one fusion and are then broken up, they are not made so thick and heavy as those used in flint-glass making, since the latter remain in the furnace for many See also:

• WEEKS, EDWIN LORD (1849-1903)

weeks . On the other hand, the chemical and physical nature of the fireclays used in the manufacture of such crucibles requires careful See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention in order to secure the best results . The furnace used for the production of optical glass is generally constructed to take one crucible only, so that the heat of the furnace may be accurately adjusted to the requirements of the particular glass under treatment . These small furnaces are frequently arranged for direct coal firing, but regenerative gas-fired furnaces are also employed . The empty crucible, having first been gradually dried and heated to a bright red heat in a subsidiary furnace, is taken up by means of massive iron See also:

• TONGS (O. Eng. Lange, M. Eng. tonge, cf. Du. tang, Ger. Zange, from base tang, to bite, cf. Gr. SaKVeiv)

tongs and introduced into the previously heated furnace, the temperature of which is then gradually raised . When a suitable temperature for the fusion of the particular glass in question has been attained, the mixture of raw materials is introduced in comparatively small quantities at a See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time . In this way the crucible is gradually filled with a mass of molten glass, which is, however, Factory See also:

• MEDIUM

Medium Partial and Relative Partial Dispersions . i Number . Name .

"D. v . Dispersion . C-D D-F F-G' C-F . C-D . C-F . D-F . C-F . F-G' . C-F . C . 644 Extra Hard Crown 1.4959 64'4 '00770 .00228 •296 .00542 •704 '00431 •56o B . 646 Boro-silicate Crown .

1.5096 63.3 •00803 .00236 .294 •00562 .700 •00446 .555 A . 605 Hard Crown P5175 6o•5 •oo856 .00252 .294 •00604 •7o6 •00484 .554 C . 577 :Medium Barium Crown 1.5738 57.9 .00990 .00293 •296 •00697 •704 .1 i552 '557 C . 579 Densest Barium Crown 1.6065 57.9 .01046 .00308 .294 •00738 •705 •00589 .563 A . 569 Soft Crown . 1.5152 56.9 .00906 •00264 •291 '00642 •708 •00517 .570 B . 563 Medium Barium Crown 1.566o 56.3 •oioo6 •00297 .295 •00709 .704 •00576 .572 B . 535 Barium Light Flint . 1.5452 53.5 •01020 •o0298 •292 .00722 •701 .00582 •570 A . 490 Extra Light Flint 1.5316 49.o •01085 .00313 •288 •00772 •711 •00630 •58o A . 485 Extra Light Flint 1.5333 48'5 •01099 .00322 '293 .00777 .707 •00640 •582 C . 474 Boro-silicate Flint .

1.5623 47'4 .01187 .00343 .289 •0o844 •711 •00693 '584 B . 466 Barium Light Flint 1.5833' 46.6 .01251 .00362 •288 .00889 •711 •00721 •576 B . 458 Soda Flint . 1.5482 45.8 .01195 .00343 .287 •00852 .713 •00690 .577 A . 458 Light Flint . 1.5472 45'8 .o11g6 •00348 •291 •00848 •7o9 •00707 •591 A . 432 Light Flint . 1.5610 43.2 .01299 .00372 .287 •00927 .713 '00770 .593 A . 410 Light Flint . 1.5760 41.0 •01404 .00402 •286 •01002 .713 '00840 '598 B . 407 Light Flint . 1.5787 40.7 .01420 •00404 .284 •oioi6 .715 •00840 •591 A .

370 Dense Flint . 1.6118 36.9 .01657 .00470 .284 •01187 '716 •01004 •6c6 A . 361 Dense Flint . 1.6214 36.1 '01722 •00491 .285 '01231 .715 .01046 •608 A . 36o Dense Flint 1.6225 36'0 .01729 •00493 •286 .01236 .715 •01054 .609 A . 337 Extra Dense Flint . 1.6469 33'7 .01917 •00541 .285 .01376 •720 •01170 •655 A . 299 Densest Flint I P7129 29.9 .02384 •00670 •281 •01714 •789 •oi661 •678 full of bubbles of all sizes . These bubbles arise partly from the air enclosed between the particles of raw materials and partly from the gaseous decomposition products of the materials themselves . In the next stage of the process, the glass is raised to a high temperature in order to render it sufficiently fluid to allow of the See also:

• COMPLETE

complete elimination of these bubbles; the actual temperature required varies with the chemical composition of the glass, a bright red heat sufficing for the most fusible glasses, while with others the utmost capacity of the best furnaces is required to attain the necessary temperature . With these latter glasses there is, of course, considerable See also:

• RISK

risk that the partial fusion and consequent contraction of the fireclay of the crucible may result in its destruction and the entire loss of the glass . The stages of the process so far described generallyoccupy from 36 to 6o hours, and during this time the constant care and watchfulness of those attending the furnace is required .

This is still more the case in the next stage . The examination of small test-pieces of the glass withdrawn from the crucible by means of an iron rod having shown that the molten mass is free from bubbles, the stirring process may be begun, the object of this manipulation being to render the glass as homogeneous as possible and to secure the absence of veins or striae in the product . For this purpose a cylinder of fireclay, provided with a square axial hole at the upper end, is heated in a small subsidiary furnace and is then introduced into the molten glass . Into the square axial hole fits the square end of a hooked iron See also:

• BAR
• BAR (0. Fr. barre, Late Lat. barra, origin unknown)
• BAR, CONFEDERATION OF
• BAR, FRANCOIS DE (1538-1606)
• BAR, THE

bar which projects several yards beyond the mouth of the furnace; by means of this bar a workman moves the fireclay cylinder about in the glass with a steady circular sweep . Although the See also:

• WEIGHT

weight of the iron bar is carried by a support, such as an overhead See also:

• CHAIN (through the O. Fr. citable, chcene, &c., from Lat. catena)

chain or a swivel roller, this operation is very laborious and trying, more especially during the earlier stages when the heat radiated from the open mouth of the crucible is intense . The men who manipulate the stirring bars are therefore changed at See also:

• SHORT, FRANCIS JOB (1857– )

short intervals, while the bars themselves have also to be changed at somewhat longer intervals, as they rapidly become oxidized, and accumulated scale would tend to fall off them, thus contaminating the glass below . The stirring process is begun when the glass is perfectly fluid at a temperature little short of the highest attained in its fusion, but as the stirring proceeds the glass is allowed to cool gradually and thus becomes more and more viscous until finally the stirring cylinder can scarcely be moved . When the glass has acquired this degree of consistency it is supposed that no fresh movements can occur within its mass, so that if homogeneity has been attained the glass will preserve it permanently . The stirring is therefore discontinued and the See also:

• CLAY (from O. Eng. claeg, a word common in various forms to Teutonic languages, cf. Ger. Klei)
• CLAY, CASSIUS MARCELLUS (1810-1903)
• CLAY, CHARLES (1801–1893)
• CLAY, FREDERIC (1838–1889)
• CLAY, HENRY (1777–1852)

clay cylinder is either See also:

• LEFT

left embedded in the glass, or by the exercise of considerable force it may be gradually withdrawn . The crucible with the semi-solid glass which it contains is now allowed to cool considerably in the melting furnace, or it may be. removed to another slightly heated furnace . When the glass has cooled so far as to become hard and solid, the furnace is hermetic-ally sealed up and allowed to cool very gradually to the ordinary temperature . If the cooling is very gradual—occupying several weeks—it sometimes happens that the entire contents of a large crucible, weighing perhaps r000 lb, are found intact as a single mass of glass, but more frequently the mass is found broken up into a number of fragments of various sizes .

From the large masses great lenses and mirrors may be produced, while the smaller pieces are used for the production of the disks and slabs of moderate See also:

• SIZE

size, in which the optical glass of See also:

• COMMERCE
• COMMERCE (Lat. commercium, from, cum, together, and rnerx, merchandise)

commerce is usually. supplied . In order to allow of the removal of the glass, the cold crucible is broken up and the glass carefully separated from the fragments of See also:

• FIRE (in O. Eng. f j'r; the word is common to West German languages, cf. Dutch vuur, Ger. Feuer; the pre-Teutonic form is seen in Sanskrit pu, pavaka, and Gr. irup; the ultimate origin is usually taken to be a root meaning to purify, cf. Lat. purus)

fire-clay . The pieces of glass are then examined for the detection of ' the grosser defects, and obviously defective pieces are rejected . As the fractured surfaces of the glass in this condition are unsuitable for delicate examination a See also:

• GOOD, JOHN MASON (1764-1827)

good See also:

• DEAL

deal of glass that passes this inspection has yet ultimately to be rejected . The next stage in the preparation of the glass is the process of moulding and annealing . Lumps of glass of approximately the right weightare chosen, and are heated to a temperature just sufficient to soften the glass, when the lumps are caused to assume the shape of moulds made of iron or fireclay either by the natural flow of the softened glass under gravity, or by pressure from suitable tools or presses . The glass, now in its approximate form, is placed iri a heated chamber where it is allowed to cool very gradually—the minimum time of cooling from a dull red heat being six days, while for " fine annealing " a much longer period is required (see above) . At the end of the annealing process the glass issues in the shape of disks or slabs slightly larger than required by the optician in each case . The glass is, however, by no means ready for delivery, since it has yet to be examined with scrupulous care, and all defective pieces must be rejected entirely or at least the defective See also:

• PART

part must be cut out and the slab remoulded or ground down to a smaller size . For the purpose of rendering this minute examination possible, opposite See also:

• PLANE

plane surfaces of the glass are ground approximately See also:

• FLAT (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

flat and polished, the faces to be polished being so chosen as to allow of a view through the greatest possible thickness of glass; thus in slabs the narrow edges are polished . It will be readily understood from the above account of the process of production that optical glass, relatively to other kinds of glass, is very expensive, the actual See also:

• PRICE
• PRICE, BARTHOLOMEW (1818–1898)
• PRICE, BONAMY (1807-1888)
• PRICE, RICHARD (1723-1791)

price varying from 3S. to 3os. per lb in small slabs or disks . The price, however, rapidly increases with the See also:

• TOTAL

total bulk of perfect glass required in one piece,. so that large disks of glass suitable for telescope objectives of wide See also:

• APERTURE (from Lat. aperire, to open)

aperture, or blocks for large prisms, become exceedingly costly .

The reason for this high cost is to be found partly in the fact that the yield of optically perfect glass even in large and successful meltings rarely exceeds 20% of the total weight of glass melted . Further, all the subsequent processes of cutting, moulding and annealing become increasingly difficult, owing to the greatly increased risk of breakage arising from either See also:

• EXTERNAL

external injury or internal strain, as the dimensions of the individual piece of glass increase . Nevertheless, disks of optical glass, both crown and flint, have been produced up to 39 in. in diameter . II . BLOWN Glass . (A) Table-See also:

• WARE

ware and Vases.—The varieties of glass used for the manufacture of table-ware and vases are the potash-lead glass, the soda-lime glass and the potash-lime glass . These glasses may be colourless or coloured . Venetian glass is a soda-lime glass; Bohemian glass is a potash-lime glass . The potash-lead glass, which was first used on a commercial scale in England for the manufacture of table-ware, and which is known as "flint" glass or "crystal," is also largely used in France, Germany and the United States . Table II. shows the typical composition of these glasses . For melting the leadless glasses, open, bowl-shaped crucibles are used, ranging from 12 to 40 in. in diameter . Glass mixtures containing lead are melted in covered, beehive-shaped crucibles holding from 12 to 18 cwt. of glass .

They have a hooded opening on one See also:

• SIDE
• SIDE (mod. Eski Adalia)

side near the See also:

• TOP (cf. Dan. top, Ger. Topf, also meaning pot)

top . This opening serves for the introduction of the glass-mixture, for the removal of the melted glass and as a source of heat for the processes of manipulation . The Venetian furnaces in the See also:

• ISLAND (O.E. ieg =isle, +land')

island of See also:

• MURANO (anc. Ammariuno)

Murano are small low structures heated with See also:

• WOOD, ANTHONY A2 (1632-1695)
• WOOD, JOHN GEORGE (1827—1889)
• WOOD, MRS HENRY [ELLEN] (1814—1887)
• WOOD, SEARLES VALENTINE (1798—188o)

wood . The heat passes from the melting furnace into the annealing kiln . In Germany, See also:

• AUSTRIA
• AUSTRIA, LOWER (Ger. Niederosterreich or Osterreich unter der Enns, " Austria below the river Enns ")
• AUSTRIA, UPPER (Ger. Oberosterreich or Osterreich ob der Enns, " Austria above the river Enns ")

Austria and the United States, gas furnaces are generally used . In England directly-heated coal furnaces are still in common use, which in many cases are stoked by mechanical feeders . There are two systems of -annealing . The manufactured goods are either removed gradually from a constant source of heat by means of a See also:

• TRAIN (M. Eng. trayn or trayne, derived through Fr. from Late Lat. trahinare, to drag, draw, Lat. trahere, cf. trail, trace, ultimately from the same source)

train of small iron trucks drawn along a See also:

• TRAMWAY

tramway by an Fe203 SiO2 . See also:

• K20

K20 . PbO . Na2O . CaO .

MgO. and Al203 . Potash-lead (flint) glass . 53.17 13.88 32.95 • • . • Soda-lime (Venetian) glass . 73.40 .. . . 18.58 5•o6 .. 2.48 Potash-lime (Bohemian) glass 71.70 12.70 .. 2.50 Io•3o .. 0 90 endless chain, or are placed in a heated kiln in which the fire is allowed gradually to die out . The second system is especially used for annealing large and heavy objects . The manufacture of table-ware is carried on by small gangs of men and boys .

In England each " gang " or " See also:

• CHAIR (in. Mid. Eng. chcere, through O. Fr. chaire or chaiere, from Lat. cathedra, later caledra, Gr. xaOi3pa, seat, cf. "cathedral"; the modern Fr. form chaise, a chair, has been adopted in English with a particular meaning as a form of carriage; chaire

chair " consists of three men and one boy . In works, however, in which most of the goods are moulded, and where less skilled labour is required, the proportion of boy labour is increased . There are generally two shifts of workmen, each shift working six hours, and the work is carried on continuously from See also:

• MONDAY (in O.E. Monandaeg, the moon's day, a translation of the Late Lat. Lunae dies, from which the French lundi is taken)

Monday See also:

• MORNING

morning until See also:

• FRIDAY (A.S. frige-dreg, fr. frige, gen. of frigu, love, or the goddess of love—the Norse Frigg,—the dreg, day; cf. Icelandic frjddagr, O.H. Ger. friatag, frigatag, mod. Ger. Freitag)

Friday morning . Directly work is suspended the glass remaining in the crucibles is ladled into water, drained and dried . It is then mixed with the glass mixture and broken glass (" cullet "), and replaced in the a 6 a, Puntee; b, See also:

• SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

spring puntee; c, blowing iron . crucibles . The furnaces are driven to a See also:

• WHITE
• WHITE, ANDREW DICKSON (1832– )
• WHITE, GILBERT (1720–1793)
• WHITE, HENRY KIRKE (1785-1806)
• WHITE, HUGH LAWSON (1773-1840)
• WHITE, JOSEPH BLANCO (1775-1841)
• WHITE, RICHARD GRANT (1822-1885)
• WHITE, ROBERT (1645-1704)
• WHITE, SIR GEORGE STUART (1835– )
• WHITE, SIR THOMAS (1492-1567)
• WHITE, SIR WILLIAM A