Linus Pauling: The...next topic that I want to talk about deals with rather weak forces, the forces
that operate between molecules, holding them together, but that are not considered
to involve valence bonds of the ordinary sort. Water is a very good example of a
substance for discussion because the properties of water are determined by a weak,
or rather weak force of a special sort, the force that is involved in the hydrogen
bond. The hydrogen bond was, I think it is proper to say that it was discovered by
Latimer and Rodebush, two professors in the University of California in 1922, and
it is a very important part of structural chemistry.
Let us consider water and ice, the ordinary form of crystalline solidified water.
I should have brought along a bucket of water and a cake of ice to illustrate this
part of the talk, but you know what water and ice are like. In particular, you know
one of the peculiar properties, almost unique properties that water and ice have.
Ice, the crystalline form, is less dense than water the liquid form. So that, water,
it is eight percent less dense, so that a cake of ice floating in water, an iceberg
floating in water, have eight percent of its volume projecting above. Well, of course,
icebergs usually float in salt water, that means, density of salt water is greater,
about twelve percent of the iceberg projects above the surface of the ocean. This
is a very valuable property that water has. It means that when ice forms on a lake
or on the ocean, it forms on top rather than on the bottom, so that in the summertime
the ice can melt. We do not have the situation that our oceans and our ponds are
solid ice from the bottom up and only a little bit of the surface melts into liquid
water in the summer.
This was, this fact was mentioned by John Tyndall, an English scientist in the Royal
Institution about seventy-five years ago, when he said this unique property of water,
of expanding on freezing, illustrates the beneficence of the creator. He, of course,
was not quite right in calling it a unique property because a few other substances
have been found that have the property of expanding on freezing. One of them is antimony.
Well, in a sense, the fact that antimony expands on freezing also illustrates the
beneficence of the creator. After they developed the invention of printing, type
is made not from lead, but from a lead-antimony alloy. Lead contracts on freezing
and if molten lead were poured into the matrix, the mold, to make the typeface, as
it shrunk away from the typeface, it would shrink in different ways so that one had
a poor impression of the matrix and the printed page would not be very clear. By
adding antimony to the lead to make an alloy that does not shrink, the antimony counteracts
the effect of the lead, the alloy does not shrink As it solidifies it gives a good
sharp impression, through the type from the matrix, and the printed page is correspondingly
beautiful and clean.
