601 
GLASS. 
purity of t)ie materials. The finefi: filiceous fand is 
fufed with purifted alkali, '".['he calces of lead aft as a 
j'lOvverfnl dux, and are much ufed in compofitions of 
this nature. Tliey give denfity, foftnefs, tranfparency, 
and a difpofftion to take a brilliant polifti. 
Tlie calces of lead have been found extremely valua¬ 
ble in forming the pure white glafs for optical inftru- 
ments. By tliis means the aberration of the refradfing 
lelefcope has been correfted, and the coloured or }'ril'- 
jna'tic rays dedi'oyed. The doftriue is this; Suppofe 
a convex lens formed of fuch glafs as afforded very 
little colorific difperfion of the rays, and a convex lens 
of fuch glafs as'afforded much of this effedt; it will 
follow, that v.’lien thefe two lenfes pofi'efs fuch a figure 
as that the concave fliall dcllroy tlie prii'm;itic colours 
produced by tlie convert, the excefs of mean refraction 
muff be in the latter ; and confequently, that tlie com¬ 
pound glafs will aft like a convex lens, and produce 
a real colourlefs focal image. -The problem of con- 
ffrufting telefcopes wliich fliall be truly achromatic, 
depends tlierefore on the perfection to vvliich tlic glafs 
can be brought. The genera! fafts refpefting glafs 
for this life are, that lead, and probably other metallic 
calces, increal'e the difperfive power of lenfes more 
than alkalis, and thefe laff more tlian earthy fluxes; 
and that an addition of alkali to glafs containing lead, 
ferves greatly to diminilh tlie mean refraction, with¬ 
out nuicli affedting the difperfive power occafioned 
by the metal. Hence it might feem eafy to compofe 
fuch glafs as tiie tlieorems of the optician demand; but 
the praftice is by no means fo ready. Regular refrac¬ 
tion demands that the medium fliould poffefs an uniform 
denfity tlirougliout, or, in otlier w'ords, lliat the parts 
of the glafs fhould be well combined together. This 
however is-feldom tlie cafe, cfpecially in the deiife me¬ 
tallic glafs. It is found, that the great fufibility of llie 
glafs of lead caufes it to flow, and occupy the interffices 
between the particles of the land before thefe are melted. 
So that fome very bright and apparently homogeneous 
glades exhibit an infinity of finall focal images of a 
candle, when exaniitied by a magnifier, which are pro¬ 
duced by rounded particles of land remaining in every 
part of the fubffance. Another fault, ftill more com¬ 
mon, confifts in veins of a different denlity from the reft, 
partly arifing from imperfedi fufion, and partly from the 
denfity of the glafs in the pots being greater the lower 
its pofition. 
Various have been the attempts to remedy tliefe de- 
fefts, particularly fince the board of longitude has of¬ 
fered a confiderable premium for this objedt. We do 
not, however, polT'efs any ample detail of thefe unfuc- 
cefsful experiments. It is generally underftood, tliat it 
is in vain to endeavour to make this glafs in finall fur¬ 
naces, becaufe the lieat in them is continually varying, 
and is either too low for the requilite fluidity, or fo 
high as.to extricate bubbles of elaf'ue matter; whereas 
a Iready heat is required for the purpole. Macquerand 
others have attempted to correct the evil by repeated 
fufions and pulverization of the glafs, and by expofing 
it to long-continued fires; but vvitliout complete fuc- 
cefs. It is faid, tliat one of the pradtices in the Englifli 
glafs-houfes confilis in lading the melted matter from 
one pot to another in the furnace. But this, on account 
of the heav)’ duty of excil'e, and tiic manner in which it 
is levied, cannot be done to any advantageous extent. 
If the glafs be fuff'ered to cool hn the pots after a per- 
fedd fulion, its parts take a fymmetrical arrangement, 
of the nature of cryffallization, by which tlie ligh.t is 
added upon in a manner independent of its figure, which 
is thougl'.t to be a great impediment to its optical ufe. 
Mr. Kier, who has had much experience in this curious 
inveffigation, is difpofed to recommend the trial of com¬ 
ponent parts different from any which liave yet been 
admitted into tiie compofitions for common glafs. It 
has been afariued, tliat Mr. Doliond cotiffrudlcd his ex- 
VoL.Vin. No.529. 
cellent tlirec-foot glaffes (which at prefent bear fo high 
a price) with one fingle pot of glafs made at the glafs- 
houfe near Wellclofe-fquare, London; and that none of 
the fame quality has fince been produced. But the pro ¬ 
prietor of that glafs-heufe afferts, that tlie original re¬ 
ceipts -and .pradtice are ftill followed in making the 
optical glafs : fo that much feems to depend on tlie ac¬ 
cidental circumftances in the. precife quantity and qua¬ 
lity of the materials, or in tlie effect or refinement of 
the matters during tlie fufion ; or perliaps in all thefe 
circumftances^ combined. See Mr. Dollondb experi-- 
ments under the article A3ERR.A’riON, vol. i. p. 17. 
M. Reaumur was the fiiTt who made any diredt expe¬ 
riments upon the converfion of glafs into porcelain, 
which takes place by the evaporation or abforption of 
its alkali, during the continuance of a degree of heat 
lefs than is fufficient to melt it. Srdlne glaffes are the 
belt for this purpofe ; and the lefs f.ilt they contain, the 
more readily they are converted. Inftances of this effedl; 
may be obferved among tlie rubbiili of brick-kilns, where 
pieces of green bottles are .'iic.t unfrequently'fubjedfed 
by accident to the requifite heat ; but tlie manual pro- 
cefs is as follows; A veffel of green glafs is to be filled 
up .to the top with a mixture of white fand and gypfuni, 
and then fet in a large crucible upon a quantity of the 
fame mixture, with which the glafs ve.Tels muff alio be 
furrouiided and covered over, and tiie whole preff'ed 
down clofe and hard. The crucible is then to be co¬ 
vered with a lid, the junftures well luted, and put into 
a potter’s kiln, where it muff; remain during'the whole 
time that the pottery is baking; after which, the glafs 
veffel will be found transformed into a milk-white porce¬ 
lain. The glafs, on fradlure, appears fibrous, as if it 
were compofed merely of filken threads laid by the fide 
of each other; it has then all'o quite loft the Imootli 
and lliining appearance of glafs, is very hard, and emits 
fparks of fire when ftriick with fteel, though not fo 
brilkly as real porcelain. Lewis obferved, tliat the 
above-mentioned materials have not exclufively tliis 
effeft upon glafs; but that powdered charcoal, foot, 
tobacco-pipe clay, and bone-afhes, produce the fame 
cliange. It is remarkable, that the furrounding fand 
becomes in fome meafure agglutinated by this procefs, 
which, if continued for a fufficient length of time, en¬ 
tirely deftroys the texture of the glafs, and renders it 
pulverulent. 
The pots ufed in the glafs-trade for melting the glafs, 
are of two kinds ; thofe for the fine white glafs are made 
of tobacco-pipe clay, which, -is firft well vvaffied, then 
calcined, and afterwards ground to a fine powder in a 
mill; which being mixed with water, is trod with tlie 
bare feet till it is of a fit confiftence to mould with tlie 
liands into the proper fliaps of the veffels. _Thofe I’or 
the green-glafs work are made of an inferior fort of 
clay, and fufficiently large to hold three or four liun- 
dred weiglit of metal. 
A very confiderable improvement in the confiru6tion 
of tlie glafs furnace, has been recently introduced by 
Mr. John Donaldfon of Briiiol, glafs-nianufafturer, and 
for which he obtained his majefty’s letters patent, dated 
March 5, 1802.—This improvement he deferibes to be 
as follows; “After the body of tlie furnace itfelf is 
conftrufted in the ufual form and diiiienfions, J proceed 
to indole the fame within another br.ilding,, adapted to 
prevent the external air fi'oni hav'iiig any acccis to the 
fire, except through the afh-ult and grate. And I make 
the faid laft-mentioned. building of fuch a figure and 
height that tlie rarefied air which hath paffed tlirougip 
tlie°fire, and ifl'ued forth from the apertures or flues 01 
the furnace into the faid furrounding building, dial 1 oc¬ 
cupy the interior fpace thereof, and produce the f.iine 
effeft upon the draught of air by whidi the fire is urged 
and maintained, as is produced by tlie heated aii in.-any 
other elevated chimney, (the fcientific caufes of whicji 
are well known, and need not be here expkuued.) And 
7 O in 
