GLASS
Perhaps (0) __one__of the commonest man-made substances is glass. In fact, it is (16)
__________________ common that few people appreciate its unique qualities. Glass is (17)
__________________ hard and transparent. It does not expand when heated, it is a very poor
conductor and it is resistant to all acids (18) __________________ hydrofluoric acid, which
dissolves it.
(19) __________________ knows exactly where or when the manufacture of glass began. (20)
__________________ is known, though, is that there were skilled glass makers in ancient Egypt
who practised it as (21) __________________ as 2000 BC, if not earlier. The Romans are also
known to (22) __________________ been particularly good (23) __________________ making
glass.
Glass is (24) __________________ from sand, an alkali and other ingredients which can (25)
__________________ colour it or change its properties. These changes are usually achieved (26)
__________________ adding certain chemicals during the process or placing thin layers of other
substances (27) __________________ two sheets of glass. The nature of the glass produced also
depends (28) __________________ the quality of the ingredients used.
Nowadays, one of the main uses of glass is in windows, which to us would seem to (29)
__________________ quite logical. However, the art of making glass was known long (30)
__________________ it was considered suitable for windows.
Mik Can GAP CAM ON RAT NHIEU
