MANUFACTURES.

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tures^' l,se ^ ' n *° a vei T dense, clear, transparent glass, remarka-v . bly soft and unctuous to the touch, fusible at a very low

red heat, and when melted acting so powerfully on allearthen vessels as to run through the common porouscrucibles in a very short time, almost like liquor througha sieve, vitrifying and corroding the bottom of the cru -cible in its passage. Litharge is therefore a most powerfulflux to all earthy mixtures, and it imparts to the glassthe valuable qualities of greater density and greaterpower of refracting rays of light, and of bearing suddenchanges from heat to cold without being so liable tocrack ; it gives also greater tenacity when red hot, andtherefore is easier to work.

(681.) Most of the finer glasses contain a considerablequantity of this oxide, particularly the London flint glass,which is used for the purposes of the table, for lustresand other ornamental works, for artificial gems, for op-tical and a variety of other purposes. Glass containingmuch lead has, however, the defect of being extremelysoft, so as to be readily scratched and injured by anyhard body which rubs against it. It is also extremelyfusible, so that thin tubes of it will bend with ease inthe flame of a candle, and will sink down to a shapeless§las s f or tnass at a very moderate heat.

pP tica l pi lr . For optical purposes another great inconvenience at->bes ‘ tending the use of lead is, that it does not become suffi-ciently intimately united with the other componentsfor the whole mass to assume a uniform density. Itwill always happen that the glass at the bottom of thepot contains a larger proportion of litharge than thatabove. This inequality subsists throughout, so thatevery stratum of the melted mass is of intermediatedensity between the stratum above and that below,which is irregular refraction in the glass when wrought;a, 'd as for optical purposes the perfectly uniform density°f the material is of the greatest importance, this defect isVer y detrimental to the use of flint glass. The difficultyobtaining a piece of flint glass sufficiently large fordie object of a large telescope, and at the same timePerfectly pure, is so great, that as large a sum as^lOOOtas been given for one piece of glass. Various attemptsnave in consequence been made to obtain a glass whichs *°uld have sufficient refraction, and yet not be liable*° diese defects, and the Royal Society undertook anextensive set of experiments, which were placed under. ,. direction of Dr. Faraday, a Report of which will be

oxide oun d hi the Philosophical Transactions for 1830.ntse i ' l! Sa- t682.) The black oxide of manganese has been used for™ an .y years in the manufacturing of glass before its( ', eciSe nature was understood. Its ancient name wasbin- S ?° a P’ which denotes its use ; namely, that of clear-wlr u . glw® from any accidental foulness of colour° r oth contract from the impurity of the alkali

„ reen ®. r mater >al3 employed, and particularly from the^ Th lr '^ e ’ w hich is owing to the presence of iron,subst* CIIC . unrlstances attending the employment of thisWhen • 'm "J »' ass -making are rather of a curious nature,"lass '/• 6C ' n a m °dcrate proportion to any simplebe ver ' m P arts a purple hue, and should its quantitycomes ^ ® r . eat l he colour is deepened until it be-j s still - near 7 black. If, while the mass thus colouredbonaceou USIOn ’ white arsenic, charcoal, or other car-follow ai !j U .V ltter P e added, an effervescence is sure toit alto'n-ether ,r becomes gradually more faint untiland transnare ' Sa PP eara ’ an d the glass is rendered clearsirable to cm? ", ” rov 'ded the green hue which it is de-nteract be considerable, the application

of a small quantity of manganese is not followed by Mechanicalany sensible tinge of purple : but the moment the pro- Processes,portion is more than sufficient for that purpose, thecolour immediately appears and must be corrected,which is performed in a very simple manner in the glass-house, by twisting into the pot of melted glass a pieceof wood, which, becoming charred by the heat, causesthe purple again to vanish ; while a slight effervescence,and an escape of numerous bubbles of air, are plainlyperceptible.

1 The oxide of manganese is of use from its propertyof powerfully assisting in the fusion of earthy bodies.

It also gives considerable density to glass, but the samedisadvantage accompanies its use. as already has beennoticed with regard to litharge ; viz. having from itsgreater specific gravity a tendency to settle to the bot-tom of the pot; in addition to which manganese actsinjuriously upon the pots, by corroding them at thebottom.

The white oxide of arsenic is another flux used pretty w)litelargely in glass-making. This substance being volatile oxide ofas it approaches the metallic state, it is of advantage to arsenic,employ nitre to oxygenate it more highly, and make itmore fixed. Arsenic is a very powerful flux, and verycheap, but it must be used only in moderation, from thecircumstance of its taking a longer time to mix intimatelywith the glass, and to become perfectly clear, than anyother of the additions commonly employed. Glasses inwhich arsenic is not thoroughly fused have a milky hue,which increases by age when the arsenic is in excess ;they have a tendency to become soft, are decomposed,and are therefore unsafe for drinking vessels. Thearsenic is constantly volatilizing from the arsenical glasswhen preparing, until it is intimately united with it, afterwhich it cannot be separated by heat or any commonmeans.

As arsenic is entirely volatilized when in contact withany carbonaceous matter, it has been made useful indispersing any substance which may remain in the glass,owing to a defect in the calcination of the alkali or anyother cause. When this happens, small lumps ofwhite arsenic are sometimes thrust down to the bottomof the glass pots, and stirred in with the contents ; andthe arsenic, meeting with the unburned carbon diffusedthrough the glass, carries it off by volatilization, and theglass is left free from the adhering colour, and thegreater part of the arsenic which was added. The mo-tion excited throughout the melting glass, by the vola-tilization of the arsenic, is also thought useful in hasten-ing the complete vitrification of the ingredients.

Nitre is used in destroying any carbonaceous matterin the ingredients with vtfhich it should be mixed beforefusion. It is also useful in destroying a red purple tingewhich is given by manganese.

On the Proportion of Ingredients in different Kinds ofGlass. J

(683.) There are five distinct kinds of glass at present Proportionmanufactured.

1. Flint or crystal glass.

2. Plate glass or glass of pure soda.

3. Crown glass, the best window glass.

4. Broad glass, a coarse window glass.

5. Bottle or coarse green glass.

Flint glass, so named because the silicious ingredient

originally employed was ground flints, is the most bril-

of ingre-dients indifferentkinds ofglass.