GLASS
Perhaps (0) __one__of the commonest man-made substances is glass. In fact, it is (16) more 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.
