1048 Transactions of tee American Institute. 



together. If suddenly cooled by dropping melted glass into water, 

 the outside suddenly assumes the rigid and more contracted form, 

 while the interior is still soft and expanded from the had conducting 

 power of the glass, "When thoroughly, cooled, the interior must still 

 retain the expanded state, so contrary to its cohesive force at common 

 temperature, and when the cohesion of the outer layer is in the least 

 disturbed, as by a scratch or slight fracture, the whole of the cohesive 

 force exerts its power to fracture the entire mass, From this fact, it 

 is necessary to cool more slowly than can l)e done in the air and the 

 process of annealing is indispensable. This consists in placing a 

 glass vessel, as soon as made, and while still hot, in one end of a long 

 annealing oven, with a fire at this end and gradually pushing to the 

 further or cold end of the oven ;• the particles of the interior and 

 exterior have then time to arrange themselves uniformly according 

 to their cohesive force at each point of temperature, until they become 

 perfectly rigid. 



Glass is very elastic, as is easily shown by any strip of window glass, 

 but more strikingly by hollow balls suspended by strings. On playing 

 with your fingers on the windows, the harmonious sounds . indicate 

 their elasticity. A glass harmonicon consists of small strips of win- 

 dow glass of different sizes, suspended on parallel strings. They 

 may be graduated to any scale ; goblets of various sizes are also some- 

 times employed in a similar manner, and are made to vibrate bypass- 

 ing the moistened finger around their upper edges. 



As has been stated, one of the various kinds of glass is the soluble 

 glass, or silicate of soda or potassa, or both combined, and on account 

 of an excess of alkali, has become a soluble salt. It is termed also 

 water iglassj and; has the formula, 2 KO, or Na 03, Si O^, according 

 to the old notation. The uses of silicate of soda are so well known 

 for the application to wood and textile fabrics, as a paint and sub- 

 stitute of dunging salt in calico printing, that it is unnecessary to 

 devote any space on this occasion. 



The BoJiemian glass is manufactured largely in Bohemia, from 

 100 parts of silica, purified pearlash, sixty parts, and carbonate of 

 lijne, sixteen parts. These three substances are fritted in a reverbe- 

 ratory oven called calcar, and while still red hot, thrown into the 

 glass pots, already in a glowing heat, and there melted, and when 

 perfectly liquid, scooped out or taken out with an iron rod. The 

 objects of fritting are to expel moisture and carbonic acid,. and pro- 

 duce a caking of the materials, which facilitates the fusion. This 



