LEA

LEA

C 30 )

fulphuric, faccholaclic, oxalic, arfenic, tartaric,muriatic, phofphoric, fulphureous, febacic, ni-tric, fluoric, citric, la&ic, acetic, boracic,prufiic, and carbonic acids.

LEAD (White) or Cerujfe. (See the fore-going article.)

LEAD (sugar of) or Acetite of Lead. (Seethe foregoing article.)

LEATHER.

■ The preparation of the fkins of animals forthe many important purpofes to which they areapplied is almofl exclufively a chemical procefsin all its branches, and as fuch will deferve ageneral notice in this place.

The art of preparing leather is unqueftion-ably one of the molt ancient known, and ispraftifed in every country on the face of theglobe with a general fimilarity of method, therefult of obvious reafoning, and long exDeri-ence.

The objects fulfilled in converting fkin intoleather are to prevent the deltrudtion by putre-faction, which unprepared lkin would undergo(though flower than with molt other parts ofanimals) and to render it ftrong, tough, anddurable, and in fome inltances impervious tomoifture. .

The recent fkin ftripped off an animal con-fifts principally of the true cutis, or membran-ous texture, the chemical compofition of whichis gelatin in a denfe ftate, but (till entirely fo-luble in water more or lefs eafily, according toits denfity. This however is penetrated withdifferent veffels for blood, lymph, oil, &c.fome of the contents of which mult of neceffityremain after the death of the animal, and iscovered on the outfide with the infenfible cuti-cle to which is attached the exterior coveringof hair, wool, fur, and the like. The chemicalcompofition of the cuticle and its investinghairy covering, appears to be condenfed albu-men, infoluble in water, and nearly incapableof itfelf of putrefa&ion, but readily feparablefrom the true flcin by flight mechanical violenceafter the adhefipn has been weakened by in-cipient fermentation or putrefaction, or the che-mical action of lime, alkalies, or acids.

The preliminary Heps of all the procefles formaking leather confift in feparating from theCutis adhering impurities and foreign matters,the animal juices retained in its pores, and thecuticle with its hairy covering, (except on thevery few cafes in which the latter is purpofelyleft on.) The true fkin being thus obtainednearly pure, and its texture fo far opened as

readily to imbibe any fubftance in which Itis macerated, is then converted into leather indifferent methods, of which there are two quitediflinft from each other: namely, that of tan-ning, or impregnating it with that peculiar ve-getable matter called tan, and tawing , in whichit imbibes alum and other falts, and afterwardsfome foluble animal matter, fuch as the whiteof egg, or fometimes blood. Thefe two pro-cefles are alfo fometimes combined, that is,firft by tawing, and afterwards finifhing with aflight tanning. A large portion of the tannedleather alfo undergoes the further operation ofcurrying , or imbuing with oil of fome kind withmuch manual labour, in order to render it fup-ple, flexible, and ftill more impenetrable bywater. As familiar examples of each, thethick fole-leather of fhoes is tanned; the whitekid leather, as it is called, for fine gloves, istawed ; the upper leather for boots and fhoes istanned and curried ; and the fine Turkey leatheris tawed, and afterwards finifhed with a flighttanning.

The flight variations in the preparation ofdifferent kinds of leather are fo numerous, thatonly fome of the leading procefles can be heredeferibed.

Tanned leather. All fkins undergo a con-fiderable preparation before they are fit to re-ceive the tanning lixivium. In mod parts ofEngland the procefs is the following for thethin fkins of cows, calves, and thofe that areufed for the more flexible kinds of leather, modof which is afterwards finifhed by currying.The hide is firft thrown into a pit with wateralone to free them from loofe dirt, blood, andother impurities. After lying there for a dayor two it is placed upon a iolid half-cylinder ofHone, called a beam, 'where it is cleared of anyadhering fat or flefh. It is then thrown into apit containing lime and water, in which it iskept for feveral days with frequent ftirring.The ufe of this is to loofen the- hair and cuticle,after which the hide is again ftretched on thebeam and the hair entirely feraped off with ablunt knife made for the purpofe. The hidebeing well freed from the lime is then put intoa pit called the majlering- pit, which is a bathcompofed of water and the dung of fome ani-mal, generally hens or pigeons, or dogs, or,where it can be had, of fea-fowl, diffufedthrough the water. The dung of horfes orcows will not anfwer, not being fufficientlyputrefeent. Here the hide remains for fomedays, more or lefs, according to its texture,