GLASS. 



short, hooked at the end ; nostrils at the 

 base linear and oblique ; feet four toed; 

 toes long 1 , slender, connected at the base 

 by a membrane : tail forked, consisting- of 

 twelve feathers. There are three species, 

 of which the principal is P. austriaca; this 

 is about as large as a black-bird, lives on 

 \vater-insectsandonworms; is found in 

 great numbers on the banks of the Rhine 

 in the neighbourhood of Strasburgh, and 

 in innumerable flocks in the deserts of 

 the Caspian Sea ; it is a bird particularly 

 clamorous and restless. See Aves, Plate 

 VIE. fig. 5. 



GLASS, a substance too well-known to 

 admit of a definition. It is a compound 

 of the fixed alkalies, or alkaline earths, 

 with silica, brought into complete fusion, 

 and then suddenly congealed. Silica, when 

 mixed with the fixed alkalies, and expo- 

 sed to a strong heat, readily enters into 

 fusion. In this state the mixture may be 

 moulded into any shape, and if suddenly 

 cooled below the temperature at which it 

 assumes the solid state, it retains the 

 transparency, and those peculiar proper- 

 ties that belong to the substance called 

 g'lass. Metallic oxides are sometimes ad- 

 ded, as well to assist in the fusion, as to 

 communicate certain colours to the mass. 

 If the melted glass be suffered to cool 

 very slowly, the different tendency of the 

 constituent parts to assume solid forms, at 

 certain temperatures, will cause them to 

 separate successively in crystals, as salts 

 held in solution in water assume the form 

 of crystals as the liquid is slowly evapo- 

 rated. But if the glass be suddenly cool- 

 ed down to the point of congelation, the 

 constituents have not time to separate in 

 succession, and the glass remains the 

 same homogenous compound as while in 

 a state effusion. Hence it should seem 

 that the vitreous quality depends entirely, 



1. upon the fusibility of the mixture, and 



2. on the suddenness with which it iscool- 

 ed down to the point of congelation. It 

 was discovered by Sir James Hall, that 

 glass always loses its vitreous state, and 

 assumes that of a stone, if more than a 

 minute or two elapses while it is cooling 

 down from complete fusion to the point 

 at which it congeals. 



There are several kinds of glass adapt- 

 ed to different uses. The best and most 

 beautiful are the flint and the plate glass. 

 These, when well made, are perfectly 

 transparent and colourless, heavy and 

 brilliant. They are composed of fixed 

 alkali, pure siliceous sand, calcined flints, 

 and litharge in different proportions. The 

 flint glass contains a large quantity of 



oxide of lead, which by certain processes 

 is easily separated. The plate glass is 

 poured in the melted state upon a table 

 covered with copper. The plate is cast 

 half an ineh thick, or more, and is ground 

 down to a proper degree of thinness, and 

 then polished. 



Crown-glass, that used for windows, is 

 made without lead, chiefly of fixed alkali 

 fused with siliceous sand, to which is add- 

 ed some black oxide of manganese, 

 which is apt to give the glass a tinge of 

 purple. 



Jiottle-glass is the coarsest and cheapest 

 kind ; into this little or no fixed alkali en- 

 ters the composition. It consists of an al- 

 kaline earth combined with alumina and 

 silica. In England it is composed of sand 

 and the refuse of the soap-boiler, which 

 consists of the lime employed in render- 

 ing his alkali caustic, and of the earthy 

 matters with which the alkali was conta- 

 minated. The most fusible is flint-glass, 

 and the least fusible is bottle-glass. 



Flint-glass melts at the temperature of 

 10 Wedgewood; crown glass at 30; 

 and bottle-glass at 47. The specific gra- 

 vity varies between 2.48 and 3.33. 



Good glass is perfectly transparent, and 

 when cold very brittle, but at a red heat 

 it is one of the most ductile bodies known, 

 and may be drawn into threads so very 

 delicate, as to become almost invisible to. 

 the human eye. It is extremely elastic, 

 and one of the most sonorous of bodies. 

 See HARMONICA. 



There are but few chemical agents 

 which have any action upon it. Mr. Davy, 

 in one of his lectures delivered in May, 

 1808, exhibited a method of decompo- 

 sing it by means of the Voltaic battery : 

 he, however, first reduced it to powder. 

 Fluoric acid, as we have seen, has a great 

 power over it, and dissolves it very quick- 

 ly (see FLUORIC ACID) : so also have 

 the fixed alkalies, when assisted by heat. 

 The continued action of hot water is said 

 to be capable of decomposing glass, which 

 it is thought will fully explain how the 

 siliceous earth was obtained by Boyle 

 and others, when they subjected water to 

 very tedious distillations in glass vessels. 

 It has also been supposed, that the defla- 

 gration of the oxygen and hydrogen gases, 

 in the formation of water, has decomposed 

 the glass, which will account for an acid 

 as part of the result. 



In making glass, the materials are com- 

 pletely fused together, and in this state 

 the hot mixture is called frit. The frit is 

 introduced into large pots made of pre- 

 pared clay, and exposed to a heat suffici- 



