, NIT

NIT ( HI )

will laft a very confiderable time before theyare fo much worn as to be unferviceable.

Nitrous acid is alfo prepared in the large wayin fome countries by diddling nitre and clay,fometimes nitre and martial vitriol.- The ap-paratus for both is the fame, namely, largeearthen pear-fhaped pots, in which the materialsare heated, and earthen globes for receivers.The pots are ranged in a long furnace in adouble row, as already defcribed. When clayis the material ufed a harfli vitriolic clay isfelected, which is firft well dried in a fmalloven, then beaten to rough powder and lifted.About five parts of nitre are taken to twelveof the clay, and when welLmixed, the mafs iswetted either with water or with the weakeftacidulous phlegm of former diftillations. Themixture is then put into the pots, and the fireis kindled and kept up with thofe precautionswhich pradtice alone can teach, and the diltilla-tion continued till all the acid has come over,and the pots are nearly red hot. The acid pro-cured in this way is generally pretty good, butmult undoubtedly be much weaker than whenmade with fulphuric acid and nitre, fincc it isnot uncommon to obtain nearly as much nitrousacid as the weight of nitre employed. Indeedit is obvious that it mult contain all the moif-ture originally left in the clay, as well as thatemployed in the mixture of the clay and nitre..The affinity that a£ts here in the produdfion ofthe acid, mull be that of the clay for the alkaliof the nitre, when ftrongly heated. Befides thisthe clays uied are confiderably vitriolic, whichalfo a fulls in the expulfion of the nitrous acid.The refidue in the retort is a hard hnlf-fufedearthy miyfs, now turned to a deep red (owing'to the compleat oxydaiion of the iron of theclay) and confuting of all the earths of theclay, the alkali of the nitre, and a little fulphatof potalh, which lall may be feparated by boilingwater. This relidue is called cement earth ofaqua forth, and is only ufed as a red earth inftaining tiles, bricks, &c.

khe procefs for making aqua fortis by martialVitriol (which is a very good method and muchU1 ed on the continent) a is very fimple. Thev itrioI is firft calcined till it falls into a whitepow er, by which it lofes nearly half its weightrp, ^ water, hut little cr none of its acid.

^ C r a c ^ n ed vitriol is then mixed with about• 'V- We ’ght of pure nitre, and the mixture

111 -onie works, into earthen, in othersP°tj, to which glafs receivers are well

m

put

iron

luted (but with a fmall vent-hole, for the tm-condenfible vapour to efcape when opened}and the fire is gradually raifed to rednefs duringwhich the acid diftills over. In this as in otherdiftillations' of this acid a fat lute made of clayand linfeed oil is generally employed. Theacid procured in this method is more ruddy andfuming than in any other, fo as to be almolkbrown and opake, and in the procefs the receiveris often found ftarred with cryftalline rays vwhich again melt down. Thefe are probablyfulphuric acid driven off by the violence of theheat and faturated with nitrous vapour,, which(as will be elfewhere mentioned) renders thisacid cryftalliaable. The refidue of this diftilla-tion is a mixture of fulphat of potalh and redoxyd of iron, which is eafily got out unlefs thenitre was mixed with common fait, in whichcafe it adheres very clofely to the veffel. Thefulphat of potalh is got out by lixivationwith hot water, and the infoluble refidue is aperfect oxyd of iron of a blood-red colour,called colcothar, and when well walhed and liftedis much ufed by the looking-glafs makers topoliffi their mirrors.

Other fubftances are occafionally ufed to dif-engage the nitrous acid from nitre, where theycan be procured cheap. Sulphat of magnefia isvery ufeful for this purpofe,, and in the neigh-bourhood of falt-works it may be fometimesobtained economically in fufheient quantity.In this cafe there appears to be firft a .doubledecompofition of the ingredients, by whichfulphat of potalh and nitrat of magnefia areproduced, and then the heat drives the nitricacid off, fo that the refidue contains both fulphatof potalh and uncombined magnefia. It is fromvery few other fubftances however that thenitric acid can be expelled by heat undecom-pofed, for when the nitrated alkalies and moltother nitrated earths are heated, the acid isdecompofed almoft at firft, and the produ&sare, not nitric acid, but the elements of thisacid difunited. Hence it is that nitre alonecannot be made to yield its acid by heat.

The diftilling veflels are made cither of glafs,or earth , or iron. The former is undoubtedlythe belt material, but is expenfive and liable toaccidents. Iron retorts when made very thickat bottom laft a confiderable time, but the acidgradually a£ts upon them and produces muchred nitrous vapour, which caules the acid to bealways exceffively fuming and high-coloured.In fome parts of Wirtembcrg it appears that they.

EncycJ. Arts k Metiers. Art- Etnt Forts.