Proceedings of tee Polytechnic Association. 1049 



glass is employed for making panes, tumblers and other articles, 

 which are characterized by their beauty when compared with flint 

 and crystal glass. They also possess greater infusibility and resist- 

 ance to chemical agents; for this reason it has become celebrated and 

 indispensable in the laboratories. 



The vial and spread glass has a similar composition to the last 

 described, and contains silica, soda, lime and sometimes potash in 

 similar proportions, as before ; but a smaller amount of soda is requi- 

 site than of potash, because soda has a lower equivalent. For spread 

 or common window glass, a considerable quantity of soda is used in 

 order to flux the materials rapidly, and the addition of salt is believed 

 to clear the glass. 



For making window panes, a lump of melted glass is taken out of 

 the pot, blown and elongated into a pear, then blown and rolled into 

 a cylinder, which is slit longitudinally on one side for its whole 

 length ; it is then placed on the smooth hearth of the flattening kiln, 

 with the slit side uppermost, and when softened by heat, is opened, 

 until it spreads out ujjon the liearth, a flattened sheet. 



Crown glass is composed of materials similar to those of the pre- 

 ceding kind, but they are generally poorer ; to 100 pai'ts silica, sixty 

 parts soda ash, eight parts potash, ten parts lime, four parts saltpeter 

 or nitrate of soda, one-eighth part of white arsenic is thrown in the 

 melting pot. The mixed materials are placed in a furnace, which is 

 of rectangular construction, containing from four to six clay pots, of 

 the capacity of half a ton of glass, and is now quickly heated up to 

 the melting point. When the first charge is melted down, the next 

 is thrown in, and so on until the pot is sufficiently filled. The tem- 

 perature is then lowered for a few hours, during which some of the 

 foreign matters subside, and the glass all rises to the top, when, after 

 raising the fire a little, it is skimmed. It is called crown glass on 

 account of the shape it assumes when broken oft' from the coal formed 

 at the end of the iron rod called the punto. 



Plate glass is composed of 100 parts silex, thirty-three parts carbon- 

 ate of soda, twenty parts carbonate of lime, and a very small propor- 

 tion of paroxyd of manganese ; say one-half part. This glass is 

 usually cast into large plates, for miri'oi-s and large panes ; all mate- 

 rials must be very pure. The arrangement for casting the ton of 

 glass into the forms are very interesting, and must be seen personally, 

 at St, Gobin, in France, or at Ravenhead, in England, to be appreci- 

 ated. 



