C I1R

CHR

( 286 )

nodules and veins in Terpentine; fome of thenodules are nearly a cubic yard in magnitude.

The method of analyfing the chromat oflead is very Ample. Vauquelin has pointedout two ways, b both of which we {hall mention,only obferving that the laft is upon the wholethe moft fatisfacfory.

Take one part of finely pulverized chromat oflead, three parts of perfectly faturated carbonatof potafh, and forty parts of water, and boil themixture together for the fpace of an hour. Asfoon as the fubftances begin to act on eachother, a brifk efFervefcence will take place, theorange colour of the lead will begin to changeto brick-red, and finally when the efFervefcencehas ceafed, there will remain at the bottom ofthe vefFel a powder of a dirty yellow colourconfilting of carbonat and chromat of lead,covered by a clear liquor of a bright goldenyellow, which is chromat of potafh. The liquorbeing poured off, and the powder well walked,fome very dilute nitric acid is to be poured on thepowder till it ceafes to efFervefce; the colour-lefs folution thus obtained is nitrat of lead,while the undecompofed refidue of chromatedlead will remain unaltered, and is afterwards tobe decompofed by a fecond digeftion with thriceits weight of carbonated potafh. The nitricfolutions of lead being mixed together are to bedecompofed by fulphat of foda, and the leadcontained in the ore is to be eftimated from thefulphat of lead thus procured. The alkalinefolutions of chromic acid being mixed with thewalkings, are to be mixed with weak nitricacid as long as any carbonic acid from the un-decompofed carbonat of potafh, is given out,and the liquor by fubfequent evaporation andcooling depofits cryftals of chromat of potafhmixed with nitre.

The fecond method of decompofing this fub-ftance is, to digeft together, at a moderate tem-perature, equal weights of chromat of lead veryfinely pulverized, ftrong and pure muriatic acid,and dillilled water, taking care to ftir the mix-ture from time to time. The chromat of leadwill be decompofed, and will change to a whitecolour, being converted for the moft part tomuriat of lead. When the acid has ceafed toad, the liquor muft be poured off, and frefhmuriatic acid (diluted as before with water) to,the amount of about one-fourth of the formerquantity is to be digefted with the refidue tillno more orange-coloured grains appear amongthe white muriat. The liquor being added tojthe former, together with the walkings, the

‘Journal des Mines, No. 34, p. 74

whole after being heated is to be placed for afew days in a cool place, that the fmall quantityof muriated lead, which it holds, may be depo-fited; when this is removed fome oxyd of filver,precipitated from its folution in nitric acid bycauftic potafli, is to be added very gradually tillthe laft portions acquire a red purple colour,thus the whole of the muriatic acid will be gotrid of, and the liquors will contain only chromicacid, which, by flow evaporation, is depofited infmall prifmatic ruby-red cryftals.

The decompofition of chromat of iron is noteffected by any means fo eafily as of chromatedlead. The action of muriatic or oxymuriaticacid upon it is very flow and imperfect, nor is aboiling folution of either pure or carbonatedpotafh attended with any better fuccefs. Themoft effectual way of proceeding according toTaffaert and Vauquelin c is to fufe in a platinacrucible the finely pounded ore with an equalweight of cauftic potafh, then to feparate allthat is foluble in water, and treat the refiduewith hot muriatic acid. By the alternate ufe ofthefe menftrua for fix or feven times each, thewhole of the ore will be taken up and diffolved.The muriatic folution being evaporated nearlyto drynefs, and then left to cool, will becomegelatinous, thus announcing the prefence of fileswhich may be obtained by rediffolving the gellyin water, in confequence of which the filex willbe precipitated; the clear liquor being thentreated with ammonia the iron will be obtainedin the ftate of oxyd. The muriatic folutionbeing thus exhaufted, the alkaline folution is tobe carefully neutralized by nitric acid, by whichmeans the alumine will be precipitated, andnothing will be left in the liquor but chromat ofpotafh and nitre, from which the chromic acidmay be obtained pure by adding nitrat of leadtill no further precipitate takes place, and thentreating the chromat of lead thus formed withmuriatic acid, as mentioned above.

Chromic acid is of an orange red colour, anda pungent metallic tafle; it is very foluble inwater, and, by gentle evaporation, cryftallizes inlengthened prifms. Like other acids it com-bines with the falifiable bafes, whence refults agenus of compound falts called chromats> thechief of which we lhall proceed to defcribe.

Chromat of Barytes is formed by mixing to-gether the aqueous folutions of barytes andchromic acid: it appears as a pale lemon-yellowprecipitate, is fparingly foluble in water, and hasno perceptible tafte. When heated, it gives outoxygen gas, and affumes a green colour.

« Ibid. No. 55, p. 5 Z%.