rROCEEDIXGS OF THE POLYTECHNIC ASSOCIATIOX. 1047 



3. Crown, or spread, a silicate .of soda and lime. 

 4- Plate, a silica of soda and lime cast into plates. 



5. Bottle, a silicate of jjotassa, lime, alumina and oxjd of iron. 



6. Crystal, silicate of potash and oxyd of lead. 



7. Flint contains more lead than the last. 



8. Strass, or paste, contains still more lead than flint. 



9. Enameled and colored glass, from all the above except ]^o. 1 

 and ZSTo. 5, 



An excess of alkali is often used in order to obtain a more fusible 

 glass, but such gh'iss is more readilj' acted upon by acids; even when 

 water is boiled in it, it will readily convert red litmus to blue, on 

 account of its alkali: caustic alkali attacks 2:1 ass bv dissolving the 

 silica, and fluohydric acid decomposes glass readily. 



As regards the physical characters of glass, it may be remarked that 

 all glass is fusible, but the temperature for different kinds is diiferent ; 

 oxyd of lead, or a larger amount of alkaline silicate imparts more 

 ready fusibility, and a similar eifect is produced by borax. Bottle 

 glass, containing oxyd of iron and aluminum and. less alkali, is more 

 difficult of fusion than other kinds. When melted glass is cooled it is 

 perfectly flexible and plastic before it is cooled down to rigidity ; the 

 softer kinds, such as flint or borax glass, when heated^ begin to be 

 plastic below a red heat ; when in the plastic state pieces will unite 

 together as firmly as if they were melted together. When glass is 

 much softened by heat, it may be readily drawn out into rods or 

 tubes, or, if passed around a revolving wheel, into minute flexible 

 threads, called thin glass hairs, and these properties cause the glass to 

 be formed into numberless shapes demanded bj' the wants of civi- 

 lized life. 



Glass conducts heat so imperfectly, that the end of a rod heated to 

 whiteness may be held with safety by the hand, within an inch or 

 two of the heated end ; the bad conducting power of glass, combined 

 with tlie cohesive force of its particles, gave rise to the manufacture 

 of Prince Rupert's dross, which are pear-shaped pieces of glass, with 

 a long thin stem, made by dropping melted glass into water ; the 

 bulb may be struck without injury, but if the smallest particle of 

 the stem be broken off, the whole drop flies into powder with explo- 

 sive noise and violence, owing to the bad conducting power oi glass, 

 combined with the cohesive force of its particles. Glass expands 

 when heated and contracts on cooling, which must be done veiy 

 slowly, in order to allow the particles to come unifarmly close 



