LAT 
tain troughs, and then put in the plates, 
which they turn once or twice a day, that 
tlicy may be equally rusted over; after this 
they are taken out, and well scowered with 
sand, and, to prevent their rusting again, 
are immediately plunged into pure water, 
in which they are to be left till the instant 
they are to be tinned or blanched, the man- 
ner of doing which is this : they flux the tin 
in a large iron crucible, which has the figure 
of an oblong pyramid with four faces, of 
which two opposite ones are less than the 
two others. The crucible is heated only 
from below, its upper part being luted with 
the furnace all round. The crucible is 
always deeper than the plates, which are to 
be tinned, are long ; they always put them 
in downright, and the tin ought to swim 
over them ; to this purpose artificers of dif- 
ferent trades prepare plates of different 
shapes; though M. Keaumur thinks them 
all exceptionable. But the Germans use 
no sort of preparation of the iron, to make„ 
it receive the tin, more than the keeping it 
always steeped in wafer, till the time ; only 
when the tin is melted in the crucible, they 
cover it with a layer of a sort of suet,' which 
is usually two inches thick, and the plate 
must pass through this before it can come 
to the melted tin. The first use of this co- 
vering is to keep the tin from burning ; for 
if any part siiould take fire, the suet would 
soon moisten it, and reduce it to its primi- 
tive state again. The blanchers say, this 
suet is a compounded matter; it is indeed 
of a black colour, but M. Reaumur sup- 
posed that to be only an artifice, to make 
it a .secret, and that it is only coloured with 
soot or the .smoke of a chimney ; but he 
found it true so far, that the common un- 
prepared suet was not sutficient ; for after 
several attempts, there was always some- 
thing wanting to render the success of the 
operation certain. The whole secret of 
blanching, therefore, was found to lie in t!;e 
preparation of tl:is suet ; and tliis, at length, 
he discovered to con-sist only in the first 
frying and burning it. This simple opera- 
tion not only gives it the colour, but puts it 
into a condition to give the iron a disposi- 
tion to be tinned, which it does surpri,singly. 
The melted tin must also have a certain de- 
gree of heat, for if it is not hot enough, it 
will not stick to the iron ; and if it is too 
liot, it will cover it with too tliin a coat, 
and the plates will have several colours, as 
red, blue, and purple, and upon the whole 
will have a cast of yellow. To prevent 
Ijiis, by knowing when the fire has a proper 
LAV 
degree of heat, they might try with small 
pieces of iron ; hut in general, use leaches 
them to know the degree, and they put in 
the iron when the tin is at a different 
standard of iieat, according as they would 
give it a thicker or thinner coat. Some- 
times ak'o they give the plates a double 
layer, as they would have them very thickly 
covered. This they do by dipping them 
into the tin, when very hot, the first time; 
and when less hot; the second. The tin 
which is to give the second coat, must be 
fresh covered with suet, and that with the 
common suet, not the prepared. 
LATUS rectv.m, in conic sections, the 
same with parameter. See Parameter. 
L.atus transtersum, in the liyperbola, 
that part of the transverse diameter, inter- 
cepted between the vertices of the two op • 
posite sections. See Hyperbola. 
LAVA, the production of jEtua, Ve.su- 
vius, Heclq, and other volcanoes, is of a 
greyish colour passing to green : it is spot- 
ted externally, and occurs poroHS, carious, 
or vesicular. Its lustre is vitreous, more or 
less glistering. It is moderately hard, brit- 
tle, easily frangible, and light. It generally 
attracts strongly the magnetic needle. It 
is easily fusible into a black, compact glass. 
It frequently encloses other fossils, espe- 
cially crystals of felspar, angite, hornblende, 
and leucite. See Volc.anic formations. 
LAVANDULA, in botany, lavender, a 
genus of the Didynamia Gymnospermia 
class and order. Natural order of Verfi- 
cillatae. Labiatoe, Jussieu. Essehtial cha- 
racter: calyx ovate, obscurely toothed, 
supported by a bracte ; corolla re-supine ; 
stamina within the tube. There are seven 
species, of which L. spica, common laven- 
der, has a shrubby stem much branched, 
frequently five or six feet high, with nume- 
rous hoary loaves, the upper ones ses.siie, 
the lower petioled ; the flowers are pro- 
duced in terminating spikes from the young 
shoots, on long peduncle.s ; the spikes arc 
composed of interrupted whorls, in which 
the flowers are from six to ten, the lower 
whorls more remote; each flower upright, 
on a short pedicel ; the usual colour of the 
corolla is blue, sometimes varying with 
white flowers ; the whole plant is covered 
with a down, composed of forked hairs. It 
is a native of the south of Europe, and has 
long been celebrated for its virtues in ner- 
vous disorders ; the officinal preparations 
of lavender, are the es.sential oil, a simple 
spirit, and a compound tincture. 
LAV.ATERA, in botany, so named from 
