GROUP IV 189
raised (Jena glass) and the use of lithium gives added strength;
replacement of calcium by lead gives a higher refracting power (flint
glass), and the SiO 2 may be partly replaced by T 2 O 5 ' (crown glass).
Addition of aluminium and boron oxides gives a glass with a low
coefficient of expansion suitable for vessels which are to be heated,
e.g. *Pyrex'. Coloured glass is made by adding an oxide of a metal
which gives a coloured silicate, e.g. cobalt (blue), iron(II) (green),
copper(I) (red).
The brittle character of glass is an obvious disadvantage, and it is
not easy to mould glass into curved shapes without loss of trans-
parency. Hence glass has, in recent years, been replaced by trans-
parent plastics; or the latter have been used to give glass resistance
to breakage by bonding together layers of glass and plastic (safety
glass). A plastic is usually composed of molecules of very high
molecular weight ("high polymers') and the name plastic is given
because many polymeric solids soften on heating (these are said to
be thermoplastic) like glass. Most polymers are composed of long
chains of carbon atoms (but see below) to which other groups may
be attached along the chain; according to the nature of these
groups, the chains may be rigid rods, kinked rods, or flexible, and
able to form coils. Moreover, during the formation of a polymer,
branching may occur, and cross-linking between the chains gives a
three-dimensional structure. Usually, extensive cross-linking leads
to hardness and complete insolubility. Polymers with little or no
cross-linking will dissolve in some organic solvents; the polymer
solid first swells in the solvent and on addition of more solvent forms
a viscous solution. The higher the molecular weight of the polymer
the greater is the viscosity. To give an otherwise hard and brittle
polymer the properties of flexibility and resistance to shrinking, a
very small amount of non-volatile solvent known as a plasticiser
may be left with the solid polymer. Alternatively, two different kinds
of chain molecules may be co-polymerised (giving something
analogous to an alloy of two different metals) to give properties
which are desirable.
Most high polymeric substances are composed of carbon chains,
but a few contain other elements, and one very important class will
now be considered.THE SILICONESIn silicon tetrachloride, SiCl 4 , chlorine atoms can be replaced by
methyl or other alkyl groups to give, for example, CH 3 SiCl 3 and
(CH 3 ) 2 SiCl 2. These two compounds are obtained when methyl