GLASS-MAKING.

SSO

indirect light upon the subject. For example, a good colorless glass has been found o'!analysis to consist of 72 parts of silica, 13 parts of potash, and 10 parts of lime, inparts. If we reduce these numbers to the equivalent ratios, we shall have the followingresults; taking the atomic weights as given by Berzelius.

1 atom

potash

= 590

14.67

i

lime

356

8-84

3

silica

1722

42-79

2

silica

1155

28-70 !

3823

95-00

•71-49

This glass would therefore have been probably better compounded with the just ato® 1proportions, to which it nearly approaches, viz. 71-49 silica, 14-67 potash, and 8-84 hm e >instead of those given above as its actual constituents.

The proportions in which silica unites with the alkaline and other oxydes are ni-dified by the temperature as above stated; the lower the heat, the less silica will en . .into the glass, and the more of the base will in general be required. If a glass "'h®contains an exeess of alkali be exposed to a much higher temperature than that of 1 -formation, a portion of the base will be set free to act upon the materials of the earthpot, or to he dissipated in fumes, until such a silicate remains as to constitute i P*, (manent glass corresponding to that temperature. Hence the same mixture of vitrifi® ^materials will yield very different results, according to the heats in which it is fusedworked in the glass-house ; and therefore the composition should always be referrible .the going of the furnace. When a species of glass which at a high temperature ft ,rn L na transparent combination with a considerable quantity of lime, is kept for some tiaie herfusion at a lower temperature, a portion of the lime unites with the silica into anotcombination of a semi-vitreous or even of a stony aspect, so as to spoil the transpareof the glass altogether. There is probably a supersilicate and a sub-silicate formedsuch cases ; the latter being much the more fusible of the two compounds. The R eamur’s porcelain produced by exposing bottle glass to a red heat for 24 hours, is an ,.pie of this species of vitreous change, in which new affinities are exercised at atemperature. An excess of silica, caused by the volatilization of alkaline mattertoo strong firing, will bring on similar appearances. . _ c0 „.

The specific gravity of glass varies from 2-3 to 3-6. • That of least specific gravity j

sists of merely silica and potash fused together; that with lime is somewhat denser,• .with oxyde of lead denser still. Plate glass made from silica, soda, and lime, lias a S Plie gravity which varies from 2-50 to 2-6 ; crystal or flint glass from 3-0 to 3-6. .

The power of glass to resist the action of water, alkalis, acids, air, and li? 1 ' 1 ’ jj e jgeneral the greater, the higher the temperature employed in its manufacture, the , sB1 (s .the proportion of its fluxes, and the more exact the equivalent ratios of its conslitu^When glass contains too much alkali, it is partially soluble in water. Most crystalis affected by having water boiled in it for a considerable time; but crown gla* s p or epoorer in alkali, and containing no lead, resists that action much longer, and is the * tr jcbetter adapted to chemical operations. The affinity of glass for water, or its ^I^ r ° en eralattraction, is also proportional to the quantity of alkali which it contains. I n “ c tivealso potash glass is more apt to become damp than soda glass, agreeably to lr,l r nof sod*hygrometric properties of these two alkalis, and also lo the smaller proportionthan of potash requisite to form glass. -Rluisb ° r

Air and light operate upon glass probably by their oxydizinsr property. & n j 0 ubt'greenish colored glasses become by exposure colorless, in consequence t j n t;edly of the peroxydizement of the iron, to whose protoxyde they owe tn ^ggesother glasses become purple red from the peroxydizement of the manganese. I \y3roS e °which contain lead, suffer another kind of change in the air, if sulphureted e pderi n 'be present; the oxyde of lead is converted into a sulphuret, with the effect oxthe surface of the glass opaque and iridescent. The more lead is in tp e jn a

quicker does this iridescence supervene. By boiling concentrated sulphuric j^eO'

glass vessel, or upon glass, we can ascertain its power of resisting or ,-jje ronS* 1strua. Good glass will remain smooth and transparent; bad glass will becoand dim. . very

The brittleness of unannealed glass by change of temperature is soD1 ^ 1 JJ (great. I have known a thick vessel to fly by vicissitudes of the atmosphere a h\gh e fdefect may be corrected by slowly heating the vessel in salt water or oil topitch consistent with the nature of these liquids, and letting it cool very s ^ o ' V Jn e rnati 0ll ‘'the limits of that tange of heat, it will, in consequence of this treatment, bearof temperature without cracking as before. . , ^ a cti° r !

It has been said that glass made from silica and alkalis alone will not resis ^ g&cie i* 1of water, but that the addition of a little lime is necessary for this eff eC