CHEMISTRY. 
192 
caufes tlieir. to furrolind the veffel, which is by that means 
more ftrongly heated ; whence this furnace gets the name 
of reverberatory. Without this circumftahce the retort 
would only be heated in its bottom, the vapours railed 
from the contained fubftance would condenl'e in the up¬ 
per part, and a continual cohobation would take place, 
without any thing palling over into the receiver ; but, 
by means of this dome, the retort is equally heated in 
every part, and the vapours being forced out, can only 
condenl'e in the neck of the retort, or in the receiver 
placed on its fide. 
Fig. 20, is Macquer’s do'cimaftic or allaying and tu- 
pelling-furnace, which lerves for allaying metals, refin¬ 
ing gold and lilver, &c. Its conftrudtion is as follows : 
A. the body of the furnace, built fquare, from 12 to 15 
inches each fide, of fire-proof clay, or thick iron-plates, 
and terminating in a truncated open pyramid. B.B.B. 
three Itrong iron-bands, fixed with fcrew's, and morticed 
in the front for grooves to the fiiders. C.C. the Aiders 
with handles. The femicircular and oblong apertures 
in front are contrived for infpedting the operation, and 
fo.placed, that when the fiiders are fliut, or meet in the 
middle, they do not reach the open part of the muffle. 
D. the afii-pit, with a femicircular aperture. Immediately 
above it is the grate. E. a femicircular aperture, of a 
proper fize to receive the muffle. F. holes through which 
iron bars are inferted for fupporting the muffle. G. a cir¬ 
cular hole, by which to manage the coals. H. a pyrami¬ 
dal cover, with a chimney, that may be added or re¬ 
moved, and occafionally heightened by an additional 
tube, in order to increafe the intenfity of heat. The 
fuel is put in at top. The dimenfions are fliewn by the 
fcale. . K. the muffle, formed of baked clay, open in 
front, and clofed behind. Its form is half a cylinder cut 
length-ways. Theopenings on the fides are to admitmore 
heat, yet without admitting allies. L. a fcorifying teft. 
M. two allaying tells, or cupels of a fmaller fize: one is 
circular; the other of an inverted, truncated, pyra¬ 
midal form. On their fpherical cavities the materials are 
placed. 
The great defideratum in the conftru6tion of furnaces 
is, to obtain an intenfe and equable heat. The failure 
of moll of them in this particular, induced the late Dr. 
Black to try many experiments for their improvement, 
and in which, by means of regilters, he was enabled to 
fucceed in a very conliderable degree. Dr. Black’s fur¬ 
nace being therefore in high ellimation, we have, in the 
Chemiltry Plate II. given an exaft reprefentation of it, 
as follows : A. fig. 1, a perlpedtive view of the furnace. 
The larger hole at top lerves for introducing the fand- 
pot, &c. and is often the mouth of the furnace. On the 
fmaller aperture, the vent or chimney B is fixed. C. 
the alh-pit, feparable, and a little than the furnace, which 
is inlerted into it, and relts on its projeiting border. 
Fig. 2, a peifpeitive fedion of the furnace. Fig. 3, a 
perfpedive view' of the alh-pit, provided with a ftnall 
door, and a damping-plate, or regilter, of fix different 
fized holes. Ti e leftion of the grate is fliewn in the fi¬ 
gure above. Fig. 4, a ring with three hooks, to be funk 
about one foot deep into tiie furnace, and on which the 
retort relts when diftilling, &c. in open fire. This fur¬ 
nace is of an elliptical form, made of Itrong wrought-iron. 
Its infide is coated in fuch a manner as to reprefent an in¬ 
verted cone. The lower opening for the grate is not in 
the middle of the bottom, but purpolely nearer to one 
end of the ellipfe, towards the larger aperture at top. 
Charcoal is the fuel molt commonly ufed in furnaces, 
It produces an intenfe heat without fmoke, but it is con- 
fumed very fall. Coke or charred pit-coal produces a 
very Itrong and lading heat. Neither of thefe produce a 
Itrong heat at a didance from the fire. Where the a£tion 
of flame is required, wood or coal mull be burned. Se¬ 
veral inconveniences attend the ule of coal, as its fuli¬ 
ginous fumesj and its aptitude to Hop the paffage of air 
by becoming fufed. It is ufed however in the reverbera¬ 
tory furnaces of glafs-houfes, and is the belt material 
where veffels are to be fupplied with a great quantity of 
heat at no great intenfity, fuch as in diltilleries, &c. 
In various experiments condufted) with furnaces, it is 
neceffary to guard the retorts from the immediate action 
of the fire, to condenfe 'and retain the vapours, which 
are expanfile, fubtile, and often corrofive ; for thefe pur- 
pofes lutes are employed. A lute, which is to clofe the 
joinings of veflels muff be as impermeable as the glafs it- 
lelf, infomuch that no matter, how fubtile foever, except 
caloric, can penetrate. To prevent glafs veffels from 
cracking by the fudden variations of heat, and to render 
them capable of preferving their figure in a higher de¬ 
gree of heat than they could fuftain without it, it is. 
ufual to put over them a covering of earth : this is call¬ 
ed a coating: it may be made of fat earth and frefh 
horfe-dung ; or clay and fine fand well worked together 
into a pafte with the addition of fome hair, fuch as brick¬ 
layers ufe; either of thofe compofitions may be laid upon 
the veffel in fucceffive thin coatings. When it is necef- 
fary to condenfe or prevent the elcape of vapours of a 
corrofive and volatile nature, the compolition called the 
fat lute is ufed. Firft take boiled linfeed oil, that is, lin— 
feed oil, which has been oxygenated and rendered drying 
by the addition of litharge, or femi-vitreous oxyd of 
lead. This is known in the fhops by the name of drying 
oil, and is thus prepared: Put into a copper-pan 100 
parts of linfeed oil with nine parts of litharge ground to 
a fine powder and (trained through filk ; place the- veffel 
in a furnace, and heat it fufficiently for the oil to diffolve 
the litharge ; ltir the mixture continually'with a wooden 
fpatula till the litharge is entirely diffolved; then take 
the veffel off the fire; leave it to cool; and preferve the 
oil thus prepared in ajar well corked. To make the fat 
lutt, take a certain quantity of clay, wafli it, then dry 
it well, and reduce it to a very fine powder or duff, which 
pafs through a filken fieve; put it into an iron mortar, 
add a fufficient quantity of the oil as above ; beat it well 
a long time, till it makes a thickilh palte, which does 
not however (tick to the hands. As this lute does not 
harden, but rather grows fofter by the heat, it requires 
to be fecured in its place by Itrips of bladder or linen 
dipped in a lute made of lime and whites of eggs, which 
is called the dry lute. Here we mult obferve, that upon 
exaftnefs and nicety in luting, depends all the fuccefs of 
chemical operations conduced by means of intenfe heat. 
Little diflies of baked earth are uled for calcining ore 
or metallic fubltances ; this is called the roajling-difn : it 
fiiould be broad and flat, very even, that it may com- 
modioufly receive all the metallic oxyds or powders 
which are put into it. The cupel, as Ihewn in Plate I. 
fig. 20, is a flat crucible, wide and broad, hollowed al- 
molt hemifpherically, in the form of a cup, whence the 
name : it is formed of the calcined bones of fheep’s trot ¬ 
ters, pulverifed, drained through lilk, and well waflied 5 
the powder is mixed with water, and cupel formed in a 
mould. 
An ingenious ftudent in chemiltry, when he has fami¬ 
liarized himfelf with the firft principles, will foon per¬ 
ceive that there are few philoiophical enquiries, if any, 
in the fmall way, that require a large apparatus of fur¬ 
naces or veffels. A tobacco-pipe is a very ufeful cruci¬ 
ble, in which a great number of operations may be per¬ 
formed in a common fire, efpecially if urged with a pair 
of good double bellows. An earthen pot, or iron ladle, 
will contain a fand-bath; and apothecaries phials, or 
Florence flafks, ferve very well for matraffes. Chafing 
diflies, or fmall iron ftoves, may be applied to ferve many 
ufeful purpofes. And the blow-pipe and fpirit-lamp, with 
a let of fmall retorts and receivers, may be adapted to 
the performance of almoft every part of experimental 
chemiltry. The black lead crucibies are alfo very convenient 
for couftru&ing a variety of moveable furnaces. They 
may 
