60(> G L A 
it turn about, upon the plate, in all direflions, until 
every part of tlieir fuperficies becomes even. When the 
fecond operation is thus finilhed, the workmen fallen 
upon another table a plate of glafs, as large as that 
which is to be polifhed, and which has already been 
ground to the fame degree of fmoothnefs. This plate 
is laid upon the otlier, and to the wooden table is fixed 
a light lapidary’s wlieel. Between the w'ooden table 
and the wheel, fqiiare pieces of thin (lone are gradually 
added, in order to increafe the weight, as tlie glafs be¬ 
comes polillied. Two workmen, by means of this wheel, 
pufh the table backwards and forwards, and caufe it to 
turn about in all diredlions, taking care that water, 
mixed with fand, is often thrown between the plates. 
By this meanSvthe plates of glafs at lafl acquire a per- 
fedtly even furface. 
After this third operation, the plates are again exa¬ 
mined, by holding them up to the light, in order to de¬ 
tect any flaws, air-bubbles, or cavities. If they have 
any cavity that can be taken away by frelh grinding, 
they are again returned to the table, and ground afrefii ; 
but if the cavity is fo deep that it cannot be removed, 
the plate is cut into various final! pieces, which ferve to 
make looking-glaiTes of a fmaller fize. 
Tlie plate is next laid upon a woollen cloth, with its 
under furface coloured red, that the degree ot tranfpa- 
rency and brilliancy to be given it, might be afeertained. 
It is then covered with a powder, compofed of tlie refi- 
duuin from the difiillation of aqua'fortis, mixed with 
vitriol of iron and common fait, wliich is nothing more 
than a very fine emery. When the plate is covered with 
the powder, the workman has a kind of brulh, made of 
felt or velvet, which is likewife covered with the pow'der, 
and with which he poliflies the plate of glafs in all di- 
reilions, until it has received the fmootli metallic brii- 
liancy required ; then it is fit for foliating or filvering. 
This operation is alfo performed fipon a table of 
flone, or of thick wood, perfectly fmooth and flat, 
and covered with grey paper thoroughly fmooth ; the 
table is moveable on an axis, in order that it may be 
inclined, fo as to caufe the mercury to run into a groove 
made round the table. On the table is fpread a Iheet 
of tin-foil, w'ell hammered after being laminated. A 
very fmall quantity of mercury is firfl poured upon the 
fheet of tin-foil, and fpread all over its furface, fo as 
juft to moirten it. A larger quantity is then poured on, 
enough to cover the flieet to a certain thicknel's all over. 
When the tin-foil is thus equally covered witli mercury, 
the application of the plate of glafs to it muft not be 
too long delayed, becaufe, in that cafe, the mercury 
would difTolve the tin-foil. As the furface of the mer¬ 
cury always becomes more or lefs calcined, a fmall quan¬ 
tity of water is poured upon it ; and, when the plate is 
to be laid on, a brufh, and afterwards a hare’s foot, is 
f iaffed over it, which entirely removes the calcined pel- 
icle, &c. A piece of parchment, extending fome lines 
above the furface of the mercury, is put upon the edge 
of the table ; and the plate, being firft made very clean 
and dry, is flipped over the parchment. It is necelTary 
that the plate fliould be prefl'ed down upon the mercury, 
that bubbles of air may not get between it and the 
metal. When the plate is fixed down upon the tin-foil, 
the table mult be gently inclined i a great part of the 
mercury then runs blf, and the plate comes in contaft 
v/ith that part which adheres to the tin-foil. A fliort 
time after, the plate is covered with a woollen cloth, 
and feveral weights are laid upon it, to increafe its ad¬ 
herence to the tin-foil. When it has been in this fltiia- 
tioq a fufficient time, the projedling edges of the tin- 
foil are cut otf, and the plate is taken up finilhed for ufe. 
This method of foliating or filvering looking-glafies 
and mirrors, is the procel's now ufed at Ildefonfo in 
Spain, which is allowed to be the fineft glals-manufac- 
tory iit Europe. The largell mirrors are there made in 
a brafs frame, one hundted and fixty-two inches long, 
G L A 
ninety-three inches wide, fix inches deep, and weighing 
nearly nine tons. 
The art of cutting glafs, by which fo much brilliancy 
and magnificence is given to cluindeliers, girandoles, 
glafs ornaments, and table fets, appears to Jiave been 
brought into England from Germany in the laft century. 
The art of engravifig external ornaments upon glafs, 
according to profeftbr Beckmann, was pra6tifcd by the 
ancient Greeks ; and was probably in ufe at a much 
earlier date among the Egyptians, who certainly appear 
to have been the firft people who found put the art of 
engraving, on precious (tones. Etching on glafs is a mo¬ 
dern invention, and is now brought to conliderable per- 
feiSlion I for which fee the article Eng r.4VING, vol. vi. 
p.813. 
Much has been faid and written on the wonderful art 
of joining broken or fraflured glafs. All that is requi- 
fite, however, is, to interpofe between the fractured 
parts, a glafs levigated like a pigment, but more due- 
tile, or of eafier fulion, than the pieces to be joined ; 
and then fiibjetling the whole to fuch a heat as will 
fufe the cementing ingredient, and make the pieces ag¬ 
glutinate, without being themfelves fufed. A compo- 
lition for cementing broken pieces of flint glafs, may be 
made by filling fome of tlie fame kind of glafs previ- 
oully reduced to a powder, adding a little red lead and 
borax, or the borax only. In this manner C. Pajot of 
Charmes joined fome broken glall'es fo firmly, tliat when 
prefented before the national inftitute at Paris, they 
would rather break clofe to the joining, than in the 
a£hial frafture. 
The .A.rmenian jewellers at Conftantinople are in poll, 
fellion of a cement of fo clear and adhelive a property, 
that by the ufe of it they not only join broken glafs 
and China, but fet diamonds and precious Hones with 
it in filver or gold ; and it is fo very (Irong that when 
applied warm, the parts never feparate. It is thus 
made : Diflblve five or fix parts, as large aspeafe, in as 
much fpirit of wine as will fuffice to render it liquid p 
in another velTel dilTolve as much ifinglafs (which has 
been previoully foaked in water till it is loft) in French, 
brandy or rum, as will amount to two ounces by mea- 
fure, and add two fmall bits of gum galbanum or am- 
moniacum, which muft be rubbed or ground till they 
are dilTolved j then mix the whole with a fufficient 
heat j keep it in a phial llopt, and when it is to be ufed 
fet it in hot water. 
The firft duties granted on glafs were by Hat. 6 & 7 
Will. III. which duties were continued for ever by a 
fubfequent a6t; but were afterwards taken oft', by Hat. 
10 & IS, Will. III. c. iS. By Hat. i9Geo. II, c. 12, an 
excife duty is laid upon glafs of 8d. per pound upon 
all crown, plate, and flint, glafs imported; 2d. per 
pound on green glafs imported, and 2s. per dozen on 
flalks and bottles imported ; and on all materials or me¬ 
tal ufed in making crown, plate, or flint, glafs, 9s. 4d. 
perhoglhead | and for making common bottles, or greea 
glafs, 2S. 4d. per hoglhead | and higher duties by fub-, 
fequent atts. 
Burning Glass. See Burning Glass, vol. iii. 
P- 533 - 
Cupping Glass, SeeSURGERY. 
Hour or Sano Glass. See FIour Glass. 
Muscovy Glass. See Mica. 
Tin Glass, the fame with bifmuth. See the article 
Chemistry, vol. iv. p. 273 ; and Bismuth, vol. iii. 
p. 69. 
Weather Glass. See Barometer, vo^. Li, p. 735. 
GLASS, 'Vitreoiis j, made of glafs t 
Get thee g/a/s eyes, 
And, like a feurvy politician, feem 
To Ise the things thou do’ft not. &hakefpeare. 
To GLASS, v.a. To fee as in a glafs 3 to reprefent 
as in a glafs or mirror. Nst in ufe { 
Methinks 
