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.
