P L A

PL A

( 234 )

intenfe white heat in order compleatly to driveoff the remaining ammoniacal muriat; this be-ing done it is to be quickly placed on a dearbright anvil and gently hammered in every di-rection by a clean hammer. This is to be re-peated feveral times, at the end of which themafs will be perfectly compaCt, and fit to belaminated or wrought in any other manner thatthe artilt chufes. It is to be obferved that whilethe platina is heating it mult lie loofe in thefire, for if it were held by the tongs they wouldinfallibly become welded to the platina, andthus greatly damage it. By the time that theplatina is thus drawn down to a compaCt bar itwill be covered by a fomewhat reddifh femivi-treous cruft proceeding chiefly from particles ofthe allies melted down upon it and extendedover its furface by the hammer. To remove this,the bar being made red hot is to be fprinkledover with pulverized glafs of borax, and thenkept for a few minutes at a white heat; whenmoderately cool it is to be plunged into dilutemuriatic acid by which the borax and othervitreous matter will be dilfolved, leaving theplatina with a perfectly clean white furface.

§. 4. Ph'jfical and Chemical Proporties of Platina .

The colour of this metal is between tin-white and iron-grey. Its duCtility and mallea-bility are both very confiderable as it may bedrawn into fine wire, and be beaten into leavesas thin as tin-foil. It pofiefles a confiderableelafticity, and in hardncis is not much inferiorto iron ; hence it may be made to receive ahigh polifh. It is fufible by galvanifm, and bythe concentrated rays of the fun, but is fcarcelyto be melted by the most powerful furnace.At a full white heat it pofiefles the property ofwelding like iron, and two pieces may thus bemade to unite with perfect accuracy. It is notoxydable by the higheft heat, or the combinedinfluence of the air and moillure ; hence it isentitled to rank among the noble or perfeftmetals.

The only acid menftrua by which it is capa-ble of being dilfolved are the oxymuriatic andnitromuriatic acids, the latter being moft com-monly employed for this purpofe. RefpeClingthe compofition of this acid and the proportionof platina that it is capable of diflblving thereis very little agreement among chemifts. Thecommon method is to prepare the acid by mix-ing together equal parts of ftrong aquafortisand fuming muriatic acid, and of this men-ftruum from 16 to 24 parts are required for

' An.Chim. xxxviii. p. 23*-

the folution of one of platina. By this modeof proceeding however there is a moft unnecef-l'ary wafte of nitric acid, and the folution al-ways contains an excels of acid. A muchmore fatisfaftory refult has been obtained byProuft/ and which appears to be the moreworthy of credit as it is deduced from feveralexperiments on a large feale. The acids em-ployed by this able chemift were commonftrong aquafortis of 35° Beaume, and muriaticacid of 15 0 . A nitro-muriatic acid compofedof thefe in the proportion of 4 oz. by weightof the former, and 12 oz. of the latter, beingdigefted on 6 oz. of platina was found to takeup 17! drachms; the folution being then con-centrated by gentle diftillation, the acid liquorthat came over', weighing 10 oz. 1 drachm, af-ter being {Lengthened by 4 oz. of muriaticacid was found to be capable of taking upabout 8f drachms more of platina. Hence itappears that 16 oz. of muriatic acid, and 4 oz.of nitric are adequate by proper managementto the folution of 2 6\ drachms (3 oz. 2.j dr.)of platina. In general a confiderable quantityof acid is loft by the imprudent application ofheat in this procefs, fo that the belt mode ofperforming this operation, efpecially when anyconfiderable quantity is employed, is to pourthe whole of the muriatic acid on the platina ina retoit with about { of the nitric acid and al-low them to aft on each other in a moderatewarmth till all effervefcence has ceafed; thento add another portion of nitric acid, and fo ontill the' whole has been made ufe of; then toproceed to diftillation, and after {Lengtheningthe prod tuft with frelh muriatic acid to digeft iton a frefh portion of platina; which beingcompleated both the folutions may be added to-gether. If crude or imperfeftly purified platinais employed, a portion of black matter will re-main undifiolved, and the colour of the folutionwill approach more or less to a deep orangebrown inftead of being orange yellow-, the colourof pure muriat of platina.

Muriat of ammonia decompofes this folutionand throws down a triple fait, the ammoniaco-muriat of platina, which if pure is yellow, butof a brick red if combined with the other me-tallic oxyds. This fait is fparingly foluble inboiling water; one ounce of it being boiledwith repeated portions of pure water requiresbetween 8 and 9 lbs. for its compleat folution.*If the fait is of a brick red colour the folutionis yellowifh red verging on green, and depofitsby {landing a little black matter, and changes to« Mouflin Ponfchkin, An.Chim.' xxiv. p. 13^