GLASS-BLOWING. 165 



Opticians, who employ glass for optical instruments, often com- 

 plain of the many defect! under which it labours. The chief of 

 these are the following : 



.S freaks. These are waved lines, often visible in glass, which 

 interrupt distinct Vision. They are probably owing sometimes to 

 want of complete fusion, which prevents the different materials 

 from combining sufficiently ; but in some cases also they may be 

 produced by the workmen lifting up, at two different times, the 

 glass which is to go to the formation of one vessel or instrument. 



Tears. These are white specks or knots, occasioned by the vi- 

 trified clay of the furnaces, or by the presence of some foreign 

 salt. 



Bubbles. These are air.bubbles which have not been allowed 

 to escape. They indicate want of complete fusion, either from too 

 little alkali, or the application of too little heat. 



Cords. These are the asperities on the surface of the glass, in 

 consequence of too little heat. 



Glass-blowing. 



The art of forming vessels of glass is termed blowing, from its 

 being in agreat measure performed by the operator blowing through 

 an iron tube, and by that means inflating a piece of glass which is 

 heated so as to become soft and exceedingly pliable. By a series 

 of the most simple and dexterous operations, this beautiful mate- 

 rial is wrought into the various utensils of elegance and utility, by 

 methods which require bat very few tools, and those of the most 

 simple construction. 



Watch-glasses are made by first blowing a hollow globe, the 

 proper radius for the glasses ; then by touching it with an iron 

 ring. This cracks out a watch.glass in an instant. The same globe 

 will make several glasses. 



Window or table-glass is worked nearly in the same manner : the 

 workman blows and manages the metal, s>o that it extends two or 

 three feet in a cylindrical form. It is then carried to the fire, and 

 the operation of blowing repeated till the metal is stretched to the 

 dimensions required, the side to which the pipe is fixed diminishing 

 gradually till it ends in a pyramidal form ; but, in order to bring 

 both ends nearly to the same diameter, while the glass continues 

 flexible, a small portion of hot metal is added to the pipe ; the 

 whole is drawn out with a pair of iron pincers, and the same end 

 is cat off with a little cold water as above. 



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