STUDY OF GLASS. Ill 



fashioned object lesson may be based on the fragments of the 

 window-pane at the time they are obtained, or they may be 

 kept until further experience has increased interest and 

 practical knowledge of glass. 



We can see objects through it, therefore we say it is 

 transparent, hence it is useful for windows, lanterns, 

 spectacles, etc. 



"We feel it and say that it is smooth and hard ; we taste it 

 and smell it and say that it is tasteless and odorless ; hence 

 it is useful for drinking vessels and the tops of certain kinds 

 of tables. 



Submitting it to comparative tests along with iron, tin and 

 wood it is seen to resist better than they the corroding action 

 of acids, hence its value for bottles and preserve jars. 



If heat be carefully applied, it stands a great deal of it 

 before yielding, but when white-hot it can be bent, and when 

 molten it can be moulded, hence its value for test-tubes and 

 hollow rods, and many kinds of scientific apparatus. 



Why is glass difficult to cut and bore 1 Why is it used to 

 cap the supports for the wires on telegraph poles'? Why 

 does it sink when dropped in water 1 



To bore glass — use the corner of a newly-broken triangular 

 file, keeping the point of contact wet with water or turpen- 

 tine, or turpentine and camphor. 



To grind the surface or edge of glass — rub one piece of 

 glass on another, keeping emery and water between them. 



To etch glass — coat it with wax or paraffin, write or draw 

 with a needle or awl, and etch by floating diluted hydrofluoric 

 acid over the engraving. Rinse the acid off with water and 

 then remove the wax. 



To silver glass, lay tin foil smoothly over it, pour some 

 mercury on the foil and leave it a few hours preferably under 

 pressure. Drain off the surplus mercury. 



