SCIENTIFIC RECREATIONS. 



Glass made on a large scale is composed of the following materials, accord- 

 ing to the kind of glass that is required. 



Flint glass (" crystal ") is very heavy and moderately soft, very white 

 and bright. It is essentially a table-glass, and was used in the construction 

 of the Crystal Palace. Its composition is pure white sea-sand, 52 parts, 

 potash 14 parts, oxide of lead, 34 parts = 100. 



Plate Glass. Crown Glass. Green (Bottle) Glass. 



Pure white sand. . 55 parts. Fine sand ... 63 parts. Sea sand . . . 80 parts. 



Soda : . '."V V . 35 Chalk .... 7 Salt 10 



Nitre . . . . . 8 Soda 30 Lime . . . . 10 



Lime . 2 



100 



100 



100 



The ingredients to be made into glass (of whatever kind it may be) are 

 thoroughly mixed together and thrown from time to time into large crucibles 



Fig. 388. Experiment with phosphuretted hydrogen. 



placed in a circle, A A (fig. 389), in a furnace resting on buttresses, BB, 

 and heated to whiteness by means of a fire in the centre, C, blown by a 

 blowing machine, the tube of which is seen at D. This 

 furnace is shown in prospective in fig. 390. The in- 

 gredients melt and sink down into a clear fluid, throw- 

 ing up a scum, which is removed. This clear glass in 

 the fused state is kept at a white heat till all air- 

 bubbles have disappeared; the heat is then lowered to 

 a bright redness, when the glass assumes a consistence 

 and ductility suitable to the purposes of the " blower." 

 Glass blowing requires great care and dexterity, and is done by twirl- 

 ing a hollow rod of iron on one end of which is a globe of melted glass, the 

 workman blowing into the other end all the time. By reheating and 

 twirling a sheet of glass is produced. Plate glass is formed by pouring the 

 molten glass upon a table with raised edges. When cold it is ground with 

 emery powder, and then polished by machinery. 



Many glass articles are cast, or " struck-up," by compression in moulds, 

 and are made to resemble cut-glass, but they are much inferior in appearance. 



Fig. 389- 

 Crucibles. 



