vi Supplement to “ Nature,” June 9, 1923 
of these qualities that gives to graphite its lubricating 
powers. If you slip on the black-leaded hearthstone, 
it is because some of the layers which are sticking to 
the sole of your shoe slide on others which cling to the 
stone. I do not know that you can find a better 
example of the relation between the external features 
of a crystal and its elementary structure. One change 
has converted the hard diamond into the soft, slipping 
graphite, and it is easy to see that the results are 
exactly what one would expect from the nature of 
the change. 
Now we may pass on to another structure which 
is much like that of the diamond, namely, that of ice 
(Fig. 3). The fundamental element of the design is again 
the fact that an atom, 
oxygen in this case, is 
surrounded symmetric- 
ally by four other atoms 
of like kind ; the latter 
making a regular tetra- 
hedron of which the 
former is the centre. 
But there is this differ- 
ence between diamond 
and ice: that in the 
latter case an atom of 
a second type, namely, 
hydrogen, is inserted 
between every pair of 
oxygen atoms. Thus, 
the immediate neigh- 
bours of each oxygen 
are fourhydrogens. As 
every hydrogen has 
only two oxygen atoms 
as neighbours, there are twice as many hydrogens as 
oxygens in the structure. That is, of course, in agree- 
ment with the known composition of water. 
Here, also, as in diamond and graphite, are to be 
found layers in which the atoms are arranged in a 
hexagonal pattern. Arctic explorers have described 
a hexagonal structure in the ice-floes ; the block break- 
ing up into hexagonal vertical columns resembling the 
pillars of the Giant’s Causeway. But the most beauti- 
_ ful ice-crystals are found in the snowflakes or in the 
frost figures on the window. The forms are of an 
intricate delicacy based always on the hexagon and 
on the angle of 60°. In the model which is illustrated 
the foreshadowing of the sixfold symmetry is shown. 
The featheriness of the snow is the outward expression 
of the lightness of the pattern, which resembles lace 
rather than a continuous structure. It is clear that 
the atoms could be packed more tightly, and that must 
have something to do with the fact that when ice is 

Fic. 3.—Ice. 
White spheres represent 
oxygen atoms, and the black spheres 
hydrogen: enough of each kind are 
drawn to show the hexagonal nature 
of the crystal. Each black ball lies 
hetween two white: each white 
touches four blacks, but in two cases 
only, on the right of the diagram, the 
full number, four, is put in. The 
distance between the centres of two 
neighbouring oxygens is known with 
accuracy, but it is not known how 
much of the intervening space is 
occupied by the hydrogen. 


compressed it tends to melt. It is not easy to under- 
stand why the atoms join together in this of all possible 
ways. It is evident that particular points in the — 
structure of one atom are linked up with corresponding — 
points in the structure of another. Such considera- — 
tions have, no doubt, to do with the internal structure 
of the atoms themselves, Mate. 
It is a curious fact that when a tetrahedral structure 
is found, as in the cases of diamond and ice, there is — 
an alternative with respect to one of the details. By 
a slight structural change somewhat difficult to describe 
in words, the tetrahedral arrangement of the diamond 
becomes the usual hexagonal arrangement of ice. 
Mr. Whipple has directed my attention to a paper 
written about a hundred years ago, in which the 
author describes ice-crystals of peculiar form which 
he had found on the wooden bridge of Queen’s College, 
Cambridge. It is possible to make out from the 
description that in this case the ice had grown as a 
diamond would do: the effect is described as one of — 
great beauty and brilliance. ; 
There is one feature of the carbon structures which _ 
is of great interest. The hexagonal ring of six atoms 
is to be found both in diamond and graphite. Now, a 
whole branch of chemistry of first-rate importance is 
concerned with the examination of substances of 
which such a ring forms the essential element of — 
design. When an atom of hydrogen is attached to 
each atom of carbon the ring with its fringe is the 
molecule of benzene. The ring is an extraordinarily — 
persistent combination. : 
Organic chemists have learnt that they can detach at — 
will one or more of the hydrogens, replacing them by — 
somewhat more complicated groups, such as the pair — 
of oxygen and hydrogen atoms called the “hydroxyl — 
group,” or the “nitro group,” consisting of one nitrogen 
and two oxygens, and soon. In this way an immense — 
number of different substances are formed of widely 
varying properties. They occur in the work of the 
dye chemists, in the manufacture of explosives, in the 
study of living organisms, and, in fact, constitute a 
most important class of bodies. Chemists have ~ 
inferred the existence of these rings by reasoning 
processes of really wonderful accuracy and power. It 
is natural to suppose that the ring which we find in 
our structures is the very ring which has been the 
concern of the organic chemist. 
We have tried to put this idea to the test, and so 
far, I think, with success. We can measure this ring 
in the diamond. It is just one hundred millionth of 
an inch across, and we can make good estimates of 
the enlargements that must result from such substitu- 
tions for the hydrogens as I have already described. 
We can then measure the space which the rings, modi- 



