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mon glafs by the iron of the falts ufed, for thecircumftance of its being green makes it pro-bable that the iron is here in the ftate of fub-oxyd, and by abforbing oxygen from the man-ganefe it prevents this latter when not in ex-cefs from mixing" any of its own red-purplecolour, and the iron itfelf, now more highlyoxygenated, pafies to the ftate in which it givesa yellow tinge, which being a lighter colour ismuch lefs perceptible, and much lefs injuresthe beauty of the glafs, This change of ironfrom green to yellow by the action of oxyd ofmanganefe alfo takes place in the liquid folutionas obferved by Scheele. On the other hand,manganefe does not alter the yellow of glafstinged by the perfe£t oxyd of iron except in asfar as it mixes its own purple with it.

It muft be owned however, that it is difficultto account for the decolouring power whicharfenic exercifes over red glafs tinged by man-ganefe on the above fuppofition of the oxyge-nation of the arfenic at the expence of themanganefe. For it is found that not only thewhite, and imperfedf oxyd of arfenic has thiseffect, but all the arfeniats, or combinations ofthe arfenic acid, which is already at the higheftftate of oxygenation, and this alfo occurs inclofe veflels; fo that it is difficult to conceivehow, when the arfenic acid is prefent, anyoxygen fhould pafs into it from the manganefe,or how in any way the latter fhould lofe oxygenwhilft the glafs lofes its colour.

The perfect deftrudtion of the tinging powerof manganefe by the addition of arfenic in anyform is (hewn as well in the Ample glafles as inthofe in which any other colouring oxyd isprefent, for when the manganefe is allowed tobe aftive the colour is a compound of the manga-nefe red, and the proper tinge belonging to theother fubftance. Thus a mixture of oxyd ofcobalt and oxyd of manganefe, in the colouringftate is of a dark purple, but on the addition ofany arfeniat, or of white arfenic, the manganefeis made inactive, and the proper cobalt bluealone appears.

Thus Klaproth b made the following experi-ments on a natural mixed ore of cobalt andmanganefe. Half an ounce of fdex, and asmuch calcined borax were combined with iograins of a mixed oxyd of cobalt and manga-nefe, and vitrified in a porcelain pot. The re-fult was a glafs of a weak cloudy amethyftinecolour.

The fame mixed oxyd was then diffolved inmuriatic acid, and precipitated by arfeniated

*> Effays,

foda. This gave a lavender-blue powder whichwas a compound of arfeniated oxyd of cobaltand manganefe, and being combined with thefame proportion of filex and borax now meltedinto a glafs of a very fine fapphire-blue colour \the manganefe therefore being made inactive,and the cobalt alone giving the tinge.

Hence the neceffity where the red colour ofmanganefe is wanted to avoid every thing arfeni-cal, and alfo nitre is generally added to keep themanganefe always at the proper ftate for im-parting its colour.

The oxyd of manganefe is a very powerfulflux for all earthy matters. This is feen in therefult of all the attempts to reduce it to areguline ftate in the ufual way of combiningwith a faline carbonaceous flux, and heating ina naked crucible. Not a particle of the oxyd isreduced in this way, but the crucible conftantiyruns down, in a heat fufficiently intenfe forthe reduftion of manganefe, together with allits contents, into a green flag. The only wayhitherto known of reducing this oxyd is to en-clofe it without any faline or earthy additionin a crucible lined with charcoal, and apply avery intenfe heat. Manganefe as well as leadgives a great denfity to glafs, and alfo like leadit always fettles fomewhat to the bottom of thepots where it accumulates, and being here outof the way of moft of the decolouring additionsit gives a purple to the glafs immediately ad-hering to the bottom, and partly corrodes thepots, fo that when thefe are worn out andbroken up, they are deeply encrufted with athick purple vitrefcent flag eaffiy feparable bythe hammer.

The white Oxyd of Arfenic is another flux ufedpretty largely in glafs-making. This fubftance(as fully mentioned under the article arfenic)is volatile in the fire in proportion as it ap-proaches the metallic ftate, and hence it is ofadvantage to employ nitre to oxygenate it morehighly, and make it more fixed. Its ufe incorrefting the purple red colour of manganefehas been juft mentioned. Arfenic is a verypowerful flux, and very cheap, but muft beufed only in great moderation, for it takes alonger time to mix intimately with glafs, andto allow it to be perfe£iiy dear, than any otherof the additions commonly employed. Glaflesin which the arfenic is not thoroughly or longenough fufed have a milky hue, which encreafesby age : when the arfenic is in excefs they tendto deliquefcence, gradually become foft, andare decompofed. They are befides unfafe as

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