Chapter V: H. S (15)

_Trade._--Natural causes, such as proximity to the richest field of
coal and ironstone in Scotland and the vicinity of hill streams of
pure water, account for much of the great development of trade in
Glasgow. It was in textiles that the city showed its earliest
predominance, which, however, has not been maintained, owing, it is
alleged, to the shortage of female labour. Several cotton mills are
still worked, but the leading feature in the trade has always been the
manufacture of such light textures as plain, striped and figured
muslins, ginghams and fancy fabrics. Thread is made on a considerable
scale, but jute and silk are of comparatively little importance. The
principal varieties of carpets are woven. Some factories are
exclusively devoted to the making of lace curtains. The allied
industries of bleaching, printing and dyeing, on the other hand, have
never declined. The use of chlorine in bleaching was first introduced
in Great Britain at Glasgow in 1787, on the suggestion of James Watt,
whose father-in-law was a bleacher; and it was a Glasgow bleacher,
Charles Tennant, who first discovered and made bleaching powder
(chloride of lime). Turkey-red dyeing was begun at Glasgow by David
Dale and George M'Intosh, and the colour was long known locally as
Dale's red. A large quantity of grey cloth continues to be sent from
Lancashire and other mills to be bleached and printed in Scottish
works. These industries gave a powerful impetus to the manufacture of
chemicals, and the works at St Rollox developed rapidly. Among
prominent chemical industries are to be reckoned the alkali
trades--including soda, bleaching powder and soap-making--the
preparation of alum and prussiates of potash, bichromate of potash,
white lead and other pigments, dynamite and gunpowder. Glass-making
and paper-making are also carried on, and there are several breweries
and distilleries, besides factories for the making of aerated waters,
starch, dextrine and matches. Many miscellaneous trades flourish, such
as clothing, confectionery, cabinet-making, bread and biscuit making,
boot and shoe making, flour mills and saw mills, pottery and
india-rubber. Since the days of the brothers Robert Foulis (1705-1776)
and Andrew Foulis (1712-1775), printing, both letterpress and colour,
has been identified with Glasgow, though in a lesser degree than with
Edinburgh. The tobacco trade still flourishes, though much lessened.
But the great industry is iron-founding. The discovery of the value of
blackband ironstone, till then regarded as useless "wild coal," by
David Mushet (1772-1847), and Neilson's invention of the hot-air blast
threw the control of the Scottish iron trade into the hands of Glasgow
ironmasters, although the furnaces themselves were mostly erected in
Lanarkshire and Ayrshire. The expansion of the industry was such that,
in 1859, one-third of the total output in the United Kingdom was
Scottish. During the following years, however, the trade seemed to
have lost its elasticity, the annual production averaging about one
million tons of pig-iron. Mild steel is manufactured extensively, and
some crucible cast steel is made. In addition to brass foundries there
are works for the extraction of copper and the smelting of lead and
zinc. With such resources every branch of engineering is well
represented. Locomotive engines are built for every country where
railways are employed, and all kinds of builder's ironwork is forged
in enormous quantities, and the sewing-machine factories in the
neighbourhood are important. Boiler-making and marine engine works, in
many cases in direct connexion with the shipbuilding yards, are
numerous. Shipbuilding, indeed, is the greatest of the industries of
Glasgow, and in some years more than half of the total tonnage in the
United Kingdom has been launched on the Clyde, the yards of which
extend from the harbour to Dumbarton on one side and Greenock on the
other side of the river and firth. Excepting a trifling proportion of
wooden ships, the Clyde-built vessels are of iron and steel, the trade
having owed its immense expansion to the prompt adoption of this
material. Every variety of craft is turned out, from battleships and
great liners to dredging-plant and hopper barges.

_The Port._--The harbour extends from Glasgow Bridge to the point
where the Kelvin joins the Clyde, and occupies 206 acres. For the most
part it is lined by quays and wharves, which have a total length of
8-1/4 m., and from the harbour to the sea vessels drawing 26 ft. can
go up or down on one tide. It is curious to remember that in the
middle of the 18th century the river was fordable on foot at Dumbuck,
12 m. below Glasgow and 1-1/2 m. S.E. of Dumbarton. Even within the
limits of the present harbour Smeaton reported to the town council in
1740 that at Pointhouse ford, just east of the mouth of the Kelvin,
the depth at low water was only 15 in. and at high water 39 in. The
transformation effected within a century and a half is due to the
energy and enterprise of the Clyde Navigation Trust. The earliest
shipping-port of Glasgow was Irvine in Ayrshire, but lighterage was
tedious and land carriage costly, and in 1658 the civic authorities
endeavoured to purchase a site for a spacious harbour at Dumbarton.
Being thwarted by the magistrates of that burgh, however, in 1662 they
secured 13 acres on the southern bank at a spot some 2 m. above
Greenock, which became known as Port Glasgow, where they built
harbours and constructed the first graving dock in Scotland. Sixteen
years later the Broomielaw quay was built, but it was not until the
tobacco merchants appreciated the necessity of bringing their wares
into the heart of the city that serious consideration was paid to
schemes for deepening the waterway. Smeaton's suggestion of a lock and
dam 4 m. below the Broomielaw was happily not accepted. In 1768 John
Golborne advised the narrowing of the river and the increasing of the
scour by the construction of rubble jetties and the dredging of
sandbanks and shoals. After James Watt's report in 1769 on the ford at
Dumbuck, Golborne succeeded in 1775 in deepening the ford to 6 ft. at
low water with a width of 300 ft. By Rennie's advice in 1799,
following up Golborne's recommendation, as many as 200 jetties were
built between Glasgow and Bowling, some old ones were shortened and
low rubble walls carried from point to point of the jetties, and thus
the channel was made more uniform and much land reclaimed. By 1836
there was a depth of 7 or 8 ft. at the Broomielaw at low water, and in
1840 the whole duty of improving the navigation was devolved upon the
Navigation Trust. Steam dredgers were kept constantly at work, shoals
were removed and rocks blasted away. Two million cubic yards of matter
are lifted every year and dumped in Loch Long. By 1900 the channel had
been deepened to a minimum of 22 ft., and, as already indicated, the
largest vessels make the open sea in one tide, whereas in 1840 it took
ships drawing only 15 ft. two and even three tides to reach the sea.
The debt of the Trust amounts to L6,000,000, and the annual revenue to
L450,000. Long before these great results had been achieved, however,
the shipping trade had been revolutionized by the application of steam
to navigation, and later by the use of iron for wood in shipbuilding,
in both respects enormously enhancing the industry and commerce of
Glasgow. From 1812 to 1820 Henry Bell's "Comet," 30 tons, driven by an
engine of 3 horse-power, plied between Glasgow and Greenock, until she
was wrecked, being the first steamer to run regularly on any river in
the Old World. Thus since the appearance of that primitive vessel
phenomenal changes had taken place on the Clyde. When the quays and
wharves ceased to be able to accommodate the growing traffic, the
construction of docks became imperative. In 1867 Kingston Dock on the
south side, of 5-1/3 acres, was opened, but soon proved inadequate,
and in 1880 Queen's Dock (two basins) at Stobcross, on the north side,
of 30 acres, was completed. Although this could accommodate one
million tons of shipping, more dock space was speedily called for, and
in 1897 Prince's Dock (three basins) on the opposite side, of 72
acres, was opened, fully equipped with hydraulic and steam cranes and
all the other latest appliances. There are, besides, three graving
docks, the longest of which (880 ft.) can be made at will into two
docks of 417 ft. and 457 ft. in length. The Caledonian and Glasgow &
South-Western railways have access to the harbour for goods and
minerals at Terminus Quay to the west of Kingston Dock, and a mineral
dock has been constructed by the Trust at Clydebank, about 3-1/2 m.
below the harbour. The shipping attains to colossal proportions. The
imports consist chiefly of flour, fruit, timber, iron ore, live stock
and wheat; and the exports principally of cotton manufactures,
manufactured iron and steel, machinery, whisky, cotton yarn, linen
fabrics, coal, jute, jam and foods, and woollen manufactures.

_Government._--By the Local Government (Scotland) Act 1889 the city
was placed entirely in the county of Lanark, the districts then
transferred having previously belonged to the shires of Dumbarton and
Renfrew. In 1891 the boundaries were enlarged to include six suburban
burghs and a number of suburban districts, the area being increased
from 6111 acres to 11,861 acres. The total area of the city and the
conterminous burghs of Govan, Partick and Kinning Park--which, though
they successfully resisted annexation in 1891, are practically part of
the city--is 15,659 acres. The extreme length from north to south and
from east to west is about 5 m. each way, and the circumference
measures 27 m. In 1893 the municipal burgh was constituted a county of
a city. Glasgow is governed by a corporation consisting of 77 members,
including 14 bailies and the lord provost. In 1895 all the powers
which the town council exercised as police commissioners and trustees
for parks, markets, water and the like were consolidated and conferred
upon the corporation. Three years later the two parish councils of the
city and barony, which administered the poor law over the greater part
of the city north of the Clyde, were amalgamated as the parish council
of Glasgow, with 31 members. As a county of a city Glasgow has a
lieutenancy (successive lords provost holding the office) and a court
of quarter sessions, which is the appeal court from the magistrates
sitting as licensing authority. Under the corporation municipal
ownership has reached a remarkable development, the corporation owning
the supplies of water, gas and electric power, tramways and municipal
lodging-houses. The enterprise of the corporation has brought its work
prominently into notice, not only in the United Kingdom, but in the
United States of America and elsewhere. In 1859 water was conveyed by
aqueducts and tunnels from Loch Katrine (364 ft. above sea-level,
giving a pressure of 70 or 80 ft. above the highest point in the city)
to the reservoir at Mugdock (with a capacity of 500,000,000 gallons),
a distance of 27 m., whence after filtration it was distributed by
pipes to Glasgow, a further distance of 7 m., or 34 m. in all. During
the next quarter of a century it became evident that this supply would
require to be augmented, and powers were accordingly obtained in 1895
to raise Loch Katrine 5 ft. and to connect with it by tunnel Loch
Arklet (455 ft. above the sea), with storage for 2,050,000,000
gallons, the two lochs together possessing a capacity of twelve
thousand million gallons. The entire works between the loch and the
city were duplicated over a distance of 23-1/2 m., and an additional
reservoir, holding 694,000,000 gallons, was constructed, increasing
the supply held in reserve from 12-1/2 days' to 30-1/2 days'. In 1909
the building of a dam was undertaken 1-1/4 m. west of the lower end of
Loch Arklet, designed to create a sheet of water 2-1/2 m. long and to
increase the water-supply of the city by ten million gallons a day.
The water committee supplies hydraulic power to manufacturers and
merchants. In 1869 the corporation acquired the gasworks, the
productive capacity of which exceeds 70 million cub. ft. a day. In
1893 the supply of electric light was also undertaken, and since that
date the city has been partly lighted by electricity. The corporation
also laid down the tramways, which were leased by a company for
twenty-three years at a rental of L150 a mile per annum. When the
lease expired in 1894 the town council took over the working of the
cars, substituting overhead electric traction for horse-power. One of
the most difficult problems that the corporation has had to deal with
was the housing of the poor. By the lapse of time and the congestion
of population, certain quarters of the city, in old Glasgow
especially, had become slums and rookeries of the worst description.
The condition of the town was rapidly growing into a byword, when the
municipality obtained parliamentary powers in 1866 enabling it to
condemn for purchase over-crowded districts, to borrow money and levy
rates. The scheme of reform contemplated the demolition of 10,000
insanitary dwellings occupied by 50,000 persons, but the corporation
was required to provide accommodation for the dislodged whenever the
numbers exceeded 500. In point of fact they never needed to build, as
private enterprise more than kept pace with the operations of the
improvement. The work was carried out promptly and effectually, and
when the act expired in 1881 whole localities had been recreated and
nearly 40,000 persons properly housed. Under the amending act of 1881
the corporation began in 1888 to build tenement houses in which the
poor could rent one or more rooms at the most moderate rentals;
lodging-houses for men and women followed, and in 1896 a home was
erected for the accommodation of families in certain circumstances.
The powers of the improvement trustees were practically exhausted in
1896, when it appeared that during twenty-nine years L1,955,550 had
been spent in buying and improving land and buildings, and L231,500 in
building tenements and lodging-houses; while, on the other side,
ground had been sold for L1,072,000, and the trustees owned heritable
property valued at L692,000, showing a deficiency of L423,050.
Assessment of ratepayers for the purposes of the trust had yielded
L593,000, and it was estimated that these operations, beneficial to
the city in a variety of ways, had cost the citizens L24,000 a year.
In 1897 an act was obtained for dealing in similar fashion with
insanitary and congested areas in the centre of the city, and on the
south side of the river, and for acquiring not more than 25 acres of
land, within or without the city, for dwellings for the poorest
classes. Along with these later improvements the drainage system was
entirely remodelled, the area being divided into three sections, each
distinct, with separate works for the disposal of its own sewage. One
section (authorized in 1891 and doubled in 1901) comprises 11 sq.
m.--one-half within the city north of the river, and the other in the
district in Lanarkshire--with works at Dalmarnock; another section
(authorized in 1896) includes the area on the north bank not provided
for in 1891, as well as the burghs of Partick and Clydebank and
intervening portions of the shires of Renfrew and Dumbarton, the total
area consisting of 14 sq. m., with works at Dalmuir, 7 m. below
Glasgow; and the third section (authorized in 1898) embraces the whole
municipal area on the south side of the river, the burghs of
Rutherglen, Pollokshaws, Kinning Park and Govan, and certain districts
in the counties of Renfrew and Lanark--14 sq. m. in all, which may be
extended by the inclusion of the burghs of Renfrew and Paisley--with
works at Braehead, 1 m. east of Renfrew. Among other works in which it
has interests there may be mentioned its representation on the board
of the Clyde Navigation Trust and the governing body of the West of
Scotland Technical College. In respect of parliamentary representation
the Reform Act of 1832 gave two members to Glasgow, a third was added
in 1868 (though each elector had only two votes), and in 1885 the city
was split up into seven divisions, each returning one member.

_Population._--Throughout the 19th century the population grew
prodigiously. Only 77,385 in 1801, it was nearly doubled in twenty
years, being 147,043 in 1821, already outstripping Edinburgh. It had
become 395,503 in 1861, and in 1881 it was 511,415. In 1891, prior to
extension of the boundary, it was 565,839, and, after extension,
658,198, and in 1901 it stood at 761,709. The birth-rate averages 33,
and the death-rate 21 per 1000, but the mortality before the city
improvement scheme was carried out was as high as 33 per 1000. Owing
to its being convenient of access from the Highlands, a very
considerable number of Gaelic-speaking persons live in Glasgow, while
the great industries attract an enormous number of persons from other
parts of Scotland. The valuation of the city, which in 1878-1879 was
L3,420,697, now exceeds L5,000,000.

_History._--There are several theories as to the origin of the name of Glasgow. One holds that it comes from Gaelic words meaning "dark glen," descriptive of the narrow ravine through which the Molendinar flowed to the Clyde. But the more generally accepted version is that the word is the Celtic _Cleschu_, afterwards written Glesco or Glasghu, meaning "dear green spot" (_glas_, green; _cu_ or _ghu_, dear), which is supposed to have been the name of the settlement that Kentigern found here when he came to convert the Britons of Strathclyde. Mungo became the patron-saint of Glasgow, and the motto and arms of the city are wholly identified with him--"Let Glasgow Flourish by the Preaching of the Word," usually shortened to "Let Glasgow Flourish." It is not till the 12th century, however, that the history of the city becomes clear. About 1178 William the Lion made the town by charter a burgh of barony, and gave it a market with freedom and customs. Amongst more or less isolated episodes of which record has been preserved may be mentioned the battle of the Bell o' the Brae, on the site of High Street, in which Wallace routed the English under Percy in 1300; the betrayal of Wallace to the English in 1305 in a barn situated, according to tradition, in Robroyston, just beyond the north-eastern boundary of the city; the ravages of the plague in 1350 and thirty years later; the regent Arran's siege, in 1544, of the bishop's castle, garrisoned by the earl of Glencairn, and the subsequent fight at the Butts (now the Gallowgate) when the terms of surrender were dishonoured, in which the regent's men gained the day. Most of the inhabitants were opposed to Queen Mary and many actively supported Murray in the battle of Langside--the site of which is now occupied by the Queen's Park--on the 13th of May 1568, in which she lost crown and kingdom. A memorial of the conflict was erected on the site in 1887. Under James VI. the town became a royal burgh in 1636, with freedom of the river from the Broomielaw to the Cloch. But the efforts to establish episcopacy aroused the fervent anti-prelatical sentiment of the people, who made common cause with the Covenanters to the end of their long struggle. Montrose mulcted the citizens heavily after the battle of Kilsyth in 1645, and three years later the provost and bailies were deposed for contumacy to their sovereign lord. Plague and famine devastated the town in 1649, and in 1652 a conflagration laid a third of the burgh in ashes. Even after the restoration its sufferings were acute. It was the headquarters of the Whiggamores of the west and its prisons were constantly filled with rebels for conscience' sake. The government scourged the townsfolk with an army of Highlanders, whose brutality only served to strengthen the resistance at the battles of Drumclog and Bothwell Brig. With the Union, hotly resented as it was at the time, the dawn of almost unbroken prosperity arose. By the treaty of Union Scottish ports were placed, in respect of trade, on the same footing as English ports, and the situation of Glasgow enabled it to acquire a full share of the ever-increasing Atlantic trade. Its commerce was already considerable and in population it was now the second town in Scotland. It enjoyed a practical monopoly of the sale of raw and refined sugars, had the right to distil spirits from molasses free of duty, dealt largely in cured herring and salmon, sent hides to English tanners and manufactured soap and linen. It challenged the supremacy of Bristol in the tobacco trade--fetching cargoes from Virginia, Maryland and Carolina in its own fleet--so that by 1772 its importations of tobacco amounted to more than half of the whole quantity brought into the United Kingdom. The tobacco merchants built handsome mansions and the town rapidly extended westwards. With the surplus profits new industries were created, which helped the city through the period of the American War. Most, though not all, of the manufactures in which Glasgow has always held a foremost place date from this period. It was in 1764 that James Watt succeeded in repairing a hitherto unworkable model of Newcomen's fire (steam) engine in his small workshop within the college precincts. Shipbuilding on a colossal scale and the enormous developments in the iron industries and engineering were practically the growth of the 19th century. The failure of the Western bank in 1857, the Civil War in the United States, the collapse of the City of Glasgow bank in 1878, among other disasters, involved heavy losses and distress, but recovery was always rapid.

AUTHORITIES.--J. Cleland, _Annals of Glasgow_ (Glasgow, 1816); Duncan,
_Literary History of Glasgow_ (Glasgow, 1886); _Registrum Episcopatus
Glasgow_ (Maitland Club, 1843); Pagan, _Sketch of the History of
Glasgow_ (Glasgow, 1847); Sir J. D. Marwick, _Extracts from the Burgh
Records of Glasgow_ (Burgh Records Society); _Charters relating to
Glasgow_ (Glasgow, 1891); _River Clyde and Harbour of Glasgow_
(Glasgow, 1898); _Glasgow Past and Present_ (Glasgow, 1884);
_Munimenta Universitatis Glasgow_ (Maitland Club, 1854); J. Strang,
_Glasgow and its Clubs_ (Glasgow, 1864); Reid ("Senex"), _Old
Glasgow_ (Glasgow, 1864); A. Macgeorge, _Old Glasgow_ (Glasgow, 1888);
Deas, _The River Clyde_ (Glasgow, 1881); Gale, _Loch Katrine
Waterworks_ (Glasgow, 1883); Mason, _Public and Private Libraries of
Glasgow_ (Glasgow, 1885); J. Nicol, _Vital, Social and Economic
Statistics of Glasgow_ (1881); J. B. Russell, _Life in One Room_
(Glasgow, 1888); _Ticketed Houses_ (Glasgow, 1889); T. Somerville,
_George Square_ (Glasgow, 1891); J. A. Kilpatrick, _Literary Landmarks
of Glasgow_ (Glasgow, 1898); J. K. M'Dowall, _People's History of
Glasgow_ (Glasgow, 1899); Sir J. Bell and J. Paton, _Glasgow: Its
Municipal Organization and Administration_ (Glasgow, 1896); Sir D.
Richmond, _Notes on Municipal Work_ (Glasgow, 1899); J. M. Lang,
_Glasgow and the Barony_ (Glasgow, 1895); _Old Glasgow_ (Glasgow,
1896); J. H. Muir, _Glasgow in 1901_.

GLASITES, or SANDEMANIANS,[1] a Christian sect, founded in Scotland by John Glas (q.v.). It spread into England and America, but is now practically extinct. Glas dissented from the Westminster Confession only in his views as to the spiritual nature of the church and the functions of the civil magistrate. But his son-in-law Robert Sandeman added a distinctive doctrine as to the nature of faith which is thus stated on his tombstone: "That the bare death of Jesus Christ without a thought or deed on the part of man, is sufficient to present the chief of sinners spotless before God." In a series of letters to James Hervey, the author of _Theron and Aspasia_, he maintained that justifying faith is a simple assent to the divine testimony concerning Jesus Christ, differing in no way in its character from belief in any ordinary testimony. In their practice the Glasite churches aimed at a strict conformity with the primitive type of Christianity as understood by them. Each congregation had a plurality of elders, pastors or bishops, who were chosen according to what were believed to be the instructions of Paul, without regard to previous education or present occupation, and who enjoy a perfect equality in office. To have been married a second time disqualified for ordination, or for continued tenure of the office of bishop. In all the action of the church unanimity was considered to be necessary; if any member differed in opinion from the rest, he must either surrender his judgment to that of the church, or be shut out from its communion. To join in prayer with any one not a member of the denomination was regarded as unlawful, and even to eat or drink with one who had been excommunicated was held to be wrong. The Lord's Supper was observed weekly; and between forenoon and afternoon service every Sunday a love feast was held at which every member was required to be present. Mutual exhortation was practised at all the meetings for divine service, when any member who had the gift of speech ([Greek: charisma]) was allowed to speak. The practice of washing one another's feet was at one time observed; and it was for a long time customary for each brother and sister to receive new members, on admission, with a holy kiss. "Things strangled" and "blood" were rigorously abstained from; the lot was regarded as sacred; the accumulation of wealth they held to be unscriptural and improper, and each member considered his property as liable to be called upon at any time to meet the wants of the poor and the necessities of the church. Churches of this order were founded in Paisley, Glasgow, Edinburgh, Leith, Arbroath, Montrose, Aberdeen, Dunkeld, Cupar, Galashiels, Liverpool and London, where Michael Faraday was long an elder. Their exclusiveness in practice, neglect of education for the ministry, and the antinomian tendency of their doctrine contributed to their dissolution. Many Glasites joined the general body of Scottish Congregationalists, and the sect may now be considered extinct. The last of the Sandemanian churches in America ceased to exist in 1890.

See James Ross, _History of Congregational Independency in Scotland_
(Glasgow, 1900). (D. Mn.)

FOOTNOTE:

[1] The name Glasites or Glassites was generally used in Scotland; in
England and America the name Sandemanians was more common.

GLASS (O. E. _glaes_, cf. Ger. _Glas_, perhaps derived from an old Teutonic root _gla-_, a variant of _glo-_, having the general sense of shining, cf. "glare," "glow"), a hard substance, usually transparent or translucent, which from a fluid condition at a high temperature has passed to a solid condition with sufficient rapidity to prevent the formation of visible crystals. There are many varieties of glass differing widely in chemical composition and in physical qualities. Most varieties, however, have certain qualities in common. They pass through a viscous stage in cooling from a state of fluidity; they develop effects of colour when the glass mixtures are fused with certain metallic oxides; they are, when cold, bad conductors both of electricity and heat, they are easily fractured by a blow or shock and show a conchoidal fracture; they are but slightly affected by ordinary solvents, but are readily attacked by hydrofluoric acid.

The structure of glass has been the subject of repeated investigations. The theory most widely accepted at present is that glass is a quickly solidified solution, in which silica, silicates, borates, phosphates and aluminates may be either solvents or solutes, and metallic oxides and metals may be held either in solution or in suspension. Long experience has fixed the mixtures, so far as ordinary furnace temperatures are concerned, which produce the varieties of glass in common use. The essential materials of which these mixtures are made are, for English flint glass, sand, carbonate of potash and red lead; for plate and sheet glass, sand, carbonate or sulphate of soda and carbonate of 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 chemistry of glass-making has been considerably widened by Dr F. O. Schott's experiments at the Jena glass-works. The commercial success of these works has demonstrated the value of pure science to manufactures.

The recent large increase in the number of varieties of glass has been chiefly due to developments in the manufacture of optical glass. Glasses possessing 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 5.0 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 action of solvents.

The fluidity of glass at a high temperature renders possible the processes of ladelling, pouring, casting and stirring. A mass of glass in a viscous state can be rolled with an iron roller like dough; can be rendered hollow by the pressure of the human breath or by compressed air; can be forced by air pressure, or by a mechanically driven plunger, to take the shape and impression of a mould; and can be almost indefinitely extended as solid rod or as hollow tube. So extensible is viscous glass that it can be drawn out into a filament sufficiently fine and elastic to be woven into a fabric.

Glasses are generally transparent but may be translucent or opaque. Semi-opacity due to crystallization may be induced in many glasses by maintaining them for a long period at a temperature just insufficient to cause fusion. In this way is produced the crystalline, devitrified material, known as Reaumur's 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 oxide of tin, oxide of arsenic, phosphate of lime, cryolite or a mixture of 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 colours with different glass-mixtures, and different oxides of the same metal may produce different colours. The purple-blue of cobalt, the chrome green or yellow of chromium, the dichroic canary-colour of uranium and the violet of manganese, are constant. Ferrous oxide produces an olive green or a pale blue according to the glass with which it is mixed. Ferric oxide gives a yellow colour, but requires the presence of an oxidizing agent to prevent reduction to the ferrous state. Lead gives a pale yellow colour. Silver oxide, mixed as a paint and spread on the surface of a piece of glass and heated, gives a permanent yellow stain. Finely divided vegetable charcoal added to a soda-lime glass gives a yellow colour. It has been suggested that the colour is due to sulphur, but the effect can be produced with a glass mixture containing no sulphur, free or combined, and by increasing the proportion of charcoal the intensity of the colour can be increased until it reaches black opacity. Selenites and selenates give a pale pink or pinkish yellow. Tellurium appears to give a pale pink tint. Nickel with a potash-lead glass gives a violet colour, and a brown colour with a soda-lime glass. Copper gives a peacock-blue which becomes green if the proportion 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 reheated develops a deep crimson-ruby colour. A similar glass, if its cooling is greatly retarded, produces throughout its substance minute crystals of metallic copper, and closely resembles the mineral called avanturine. There is also an intermediate stage in which the glass has a rusty red colour by reflected light, and a purple-blue colour by transmitted light. Glass containing gold behaves in almost precisely the same way, but the ruby glass is less crimson than copper ruby glass. J. E. C. Maxwell 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 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 oxygen at a high temperature, such as manganese dioxide or arsenic trioxide. With the same object, red lead and 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 water is poured into a glass vessel, the vessel frequently breaks, on account of the unequal expansion of the inner and outer layers. If in the 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 tallow. The surface of the glass was hardened, but the inner layers remained in unstable equilibrium. Directly the crust was pierced the whole mass was shattered into minute fragments. In all branches of glass manufacture the process of "annealing," i.e. cooling the manufactured objects sufficiently slowly to allow the constituent particles to 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 constant source of heat, or by placing them in a heated kiln and allowing the heat gradually to die out.

The furnaces (fig. 15) employed for melting glass are usually heated with gas on the "Siemens," or some similar system of regenerative heating. In the United States natural gas is used wherever it is available. In some English works coal is still employed for direct heating with various forms of mechanical stokers. Crude petroleum and a thin tar, resulting from the process of enriching water-gas with petroleum, have been used both with compressed air and with steam with considerable success. Electrical furnaces have not as yet been employed for ordinary glass-making on a commercial scale, but the electrical plants which have been erected for melting and moulding 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 radiation pyrometer this difficulty is obviated, as the instrument may be placed at a considerable distance from the furnace. The radiation passing out from an opening in the furnace falls upon a concave mirror in a 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 galvanometer, the scale of which is divided and figured so that the temperature may be directly read. (See THERMOMETRY.)

In dealing with the manufacture of glass it is convenient to group the various branches in the following manner:

_Manufactured Glass._

I. Optical Glass
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II. Blown Glass
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A. Table glass. B. Tube. C. Sheet D. Bottles.
Special glasses and crown
for thermometers, glass.
and other special
glasses.

III. Mechanically Pressed Glass
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A. Plate and rolled plate glass. B. Pressed table glass.