“The differ-
ence between solids and liquids is, then, that in a solid the atoms are arranged in
some kind of an array, called a crystalline array, and they do not have a random
position at long distances; the position of the atoms on one side of the crystal
is determined by that of other atoms millions of atoms away on the other side of
the crystal. Figure 1-4 is an invented arrangement for ice, and although it con-
tains many of the correct features of ice, it is not the true arrangement. One of the
correct features is that there is a part of the symmetry that is hexagonal. You can
see that if we turn the picture around an axis by 120°, the picture returns to itself.
So there is a symmetry in the ice which accounts for the six-sided appearance of
snowflakes. Another thing we can see from Fig. 1-4 is why ice shrinks when it
melts. The particular crystal pattern of ice shown here has many "holes" in it,
as does the true ice structure. When the organization breaks down, these holes
can be occupied by molecules. Most simple substances, with the exception of
water and type metal, expand upon melting, because the atoms are closely packed
in the solid crystal and upon melting need more room to jiggle around, but an
open structure collapses, as in the case of water.”