T I N

TIN

( 4S (5 )

fey itfelf or mixed with the oxyd of lead isfufed with the faline and earthy vitreicent mix-tures. it gives a beautiful opake white, as wehave already mentioned under the article Ena-mel. It is doubtful however whether in thisprocefs th; oxyd of tin is brought to compleatfufion, and whether the opacity does not arifefrom this circumftance.

It is fotnewhat doubtful whether there is anydiiiintt intermediate oxyd of tin between thetwo that we have mentioned, or whether thefucceflive fnades of colour which the firil oxydafltunes during its compleat calcination, are notowing to a mixture of the perfect with theimperfect oxyd.

A preparation of the oxyd of tin is made forthe purpoleof giving the high eft- polifh to fteel,and to glafs and metal mirrors, which is calledTin Putty . It is prepared according to Beaurnein the following way.

Some tin is melted in an iron veffel with alow red heat, and the oxyd that forms on thefurface is fucceflively removed till enough of itis procured. This is then fpread on a-red-hotmuffle, and heated for half an hour, with fre-quent ftirring, to compleat the calcination ofany particles of tin that may be entangled inthe oxyd. When cold it is powdered andfifted, and the finer part is again calcined forfix or feven hours on a muffle with a ftrongerheat, till it becomes almoft white, and confi-derably hard, and in this ftate it forms the tinputty.

This fubftance is made in this country in afimilar way. For the fineft putty the purellgrain tin is employed, which is calcined in amuffle, finely levigated, and waffled. This isnearly white, hut the ordinary and cheaperforts are browner, and are made by calciningold pewter, or elfe a mixture of tin and lead,or any other alloy of thefe metals, which whenin mixture oxydate ftill more readily thancither of the metals feparately, and will eafflytake fire as foon as the heat is railed to rednefs.As the oxyd of lead is very fufible, and theoxyd of tin very little fo, the fubi'equent calcin-ation in this cafe is probably made at a lowerheat than when pure tin is ufed, otherwife thewhole would run into a denfe glafs. This pre-paration mull not be confounded with glazier’sp nty, which is only chalk beat up with linfeedoil. , . . ,

Tin is readily oxydated by nitre in thefollowing way. Project a little- of .this fait'ontin made moderately red-hot in a crucible. Avery ftrong deflagration enfues, with a brilliant

white flame and a denfe white fmoke, whichlatter confifls ot part of the oxyd of tin volati-lized, and will fettle on the adjacent fur-faces in the form of a fine white powder.Continue to project finall quantities of nitre onthe tin as long as any effeht is produced (theheat being kept up to full rednefs) by whichthe whole of the metal Is oxydated in the higheftdegree. When cold, difl'olve out all the folublepart of the contents of the vtflel, and a verypure white oxyd of tin will be left behind,which is lometimes ufed as the bale for opakeenamel colours, though this in general is amixture of the oxyds 1 of tin and load made bycalcination. Tlie foluble part of the refiduefrom the deflagration of tin and nitre, confifls ofthe undecompofed nitre, if any, together withthe potalh of the- portion which has been de-compofed, in a cauftic ftate, and holding a con-fiderable quantity of the fame oxyd of tin infolution, which may be feparated by faturationwith an acid. This oxyd of tin by nitre maybe prepared rather more conveniently by previ-oufly mixing together one part of tin filingswith three parts of nitre, and projeffing themixture by fpoonfuls into a crucible made veryhot. The grey oxyd with a fmaller quantity oFnitre will anfwer equally well.

It does not appear that an exa£t analyfis hasbeen made of the different oxyds of tin formedby the mere adlion of fire and air, but thewhite oxyd, which is that formed by compleatcombuftion at a ftrong heat, is probably at thehigheft ftate of oxygenation, and therefore inits compofition refembles the perfect oxydmade by nitric acid, which will be prefentlymentioned.

A very marked difference is found in thechemical properties of the higher and loweroxyds of tin. The grey oxyd (or sub-oxyd as itmay be termed for diflindtion) made by calcin-ation at a low heat, takes fire when brought toa full red, and affumes a white colour withincreafe of oxydation—it detonates with nitre,and is thus converted to the higher oxyd—itdiflolvcs in muriatic acid with emiffion of hy-drogen, and in dilute nitric acid—when in folu-tion it gives a black brown precipitate withfulphuret of potafh, and a black with corrofivemercurial muriat—and it is reduced to the me-tallic ftate again with the utmoft eafe, by beingmoderately heated with pitch, charcoal, or anycarbonaceous matter. On the other hand, thewhite or perfeEl oxyd is not further changed bythe action of fire except being fufed by it whenvery intenfe, during which it again lofes part