LAT 



LAV 



of arr 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 1 

 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 pur- 

 pose artificers of different trades prepare 

 plates of different shapes ; though M. 

 Reaumur thinks them all exceptionable. 

 But the Germans use no sort of prepara- 

 tion of the iron, to make it receive the tin, 

 more than the keeping it always steeped 

 in water 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 tltis covering is to keep the tin from 

 burning; for if any part should take fire, 

 the suet would soon moisten it, and re- 

 duce it to its primitive state again. The 

 blanchers say, this suet is a compounded 

 matter ; it is indeed of a black colour, but 

 M. Reaumur supposed 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 unprepared suet was 

 not sufficient ; for after several attempts, 

 there was always something wanting to 

 render the success of the operation cer- 

 tain. This whole secret of blanching, 

 therefore, was found to lie in the prepa- 

 ration of this suet; and this, at k-ngth, he 

 discovered to consist only in the first fry- 

 ing and burning it. This simple opera- 

 tion not only gives it the colour, but puts 

 it into a condition to give the iron a dis- 

 position to be tinned, which it does sur- 

 prisingly. The melted tin must also 

 have a certain degree of heat, for if it is 

 not hot enough, it will not stick to the 

 iron ; and if it is too hot, it will cover it 

 with too thin 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 this, by 

 knowing when the fire has a proper de- 

 gree of heat, they might try with small 

 pieces of iron -, but in general, use teach- 

 es them to know the degrve, and they 

 put in the iron when th tin is at a differ- 

 ent standard of heat, according us they 

 would give it a thicker or a thinner coat. 

 Sometimes also 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 rectum, in conic sections, the 

 same with parameter. See PAUAMKTKR. 



LATUS transversum, in the hyperbola, 

 that part of the transverse diameter, in- 

 tercepted between the vertices of the two 

 opposite sections. See HYPERBOLA. 



LAVA, the production of ./Etna, Vesu- 

 vius, Hecl-i, and other volcanos, is of a 

 greyish colour passing to green: it is 

 spotted externally, and occurs porous, 

 carious, or vesicular. Its lustre is vitreous, 

 more or less glistering. It is moderately 

 hard, brittle, 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, especially crystals of felspar, 

 augite, hornblende, and leucite. See VOL- 

 CANIC formations, 



LAVANDULA, in botany, la-vender, a 

 genus of the Didynamia Gymnospermia 

 class and order. Natural order of Verti- 

 cillatz. Labiatae, Jussieu. Essential 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 nu- 

 merous hoary leaves, the upper ones ses- 

 sile, the lower petioled ; the flowers are 

 produced in terminating spikes from the 

 young shoots, on long peduncles ; the 

 spikes are composed of interrupted 

 whorls, in which the flowers are from six 

 to ten, the lower whorls more remote ; 

 each flower upright, on a short pedicle ; 

 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 nervous dis- 

 orders ; the officinal preparations of lav- 

 ender, are the essential oil, a simple 

 spirit, and a compound tincture. 



LAVATERA, in botany, so named from 

 Lavater, a physician at Zurich ; a genus 

 of the Monadelphia Polyandria class and 

 order. Natural order 'of Columniferae. 

 Malvaceae, Jussieu. Essential character ; 

 calyx double, outer tiifid; arils very many, 

 one-seeded, There are nine species, of 

 which L. arborea lavatera, or mallow -tree, 

 rises in gardens, with a strong, thick 

 stalk, frequently to the height of eight or 

 ten feet ; in its wild state, not more than 

 four or five ; leaves alternate, cordate, 

 roundish, seven-angled, the angles blunt, 



