
- We may now go on to consider individual cases. It 
is, perhaps, natural to a new form of inquiry to deal 
with particular instances of its application as they | 
have been so far made, rather than to attempt broad 
generalisations. As we consider each case let us look 
at it from the point of view already emphasised. Let 
us try to see how the properties of the whole crystal 
depend strictly upon and are, indeed, an index to the 

Fic. t.—Diamond; showing how each carbon atom, represented—only 
diagrammatically —by a black ball, lies at the centre of gravity of its 
four negrest neighbours. 
properties of the atoms and atomic combinations of 
which it is made. : 
The diamond is, perhaps, the best to begin with. 
Its unique qualities dispose us to expect a structure 
which is equally distinguished, and so it turns out to 
be. The structure is figured in the accompanying 
sketch (Fig. 1). It may look at first sight somewhat 
complicated, but when it is examined closely it is found 
that the whole story is told in one sentence. Each atom 
has four neighbours regularly disposed about it. In 
other words, the four make a regular tetrahedron, and 
the first atom is at the centre of it. In the arrangement 
so determined by X-ray analysis we recognise at once 
an agreement with one of the most important deduc- 
tions of the chemist, the so-called tetravalency of the 
carbon atom, which means a tendency to associate 
itself with its neighbours by four bonds of equal 
strength. The hardness and strength of the diamond 
are based on the simplicity and regularity of this 
tetrahedron arrangement, and in addition on the 
strength of the tie between atom and atom. We find 
that atoms are fastened together by bonds of two or 
three different types; the one here illustrated is the 
strongest of all. Every atom, we know nowadays, 
consists of a central core, which is positively electrified, 
and of a sufficient number of negative bodies of a 
second kind called electrons to balance the positive 
charge on the core. The diamond is an example of 
many cases where neighbouring atoms share electrons 
and build them each into their own structure. It is 
somewhat analogous to the sharing of party-walls by 
eit 
y 
. . a 
Supplement to “ Nature,” June 9, 1923 Vv 


‘the houses of a terrace. Yet it can be seen that the 
‘structure is obviously weaker in certain directions than 
in others. Such are the horizontal planes in the figure. 
‘These are called the cleavage planes. The diamond- 
worker takes advantage of the fact, using it skilfully 
instead of grinding. An excellent example is to be 
found in the exhibit of the Crown Jewels in the Tower, 
where the manner of cleaving one of the great diamonds 
is shown. There is a second plane of cleavage, which 
is only used by workmen of the greatest skill, as it is 
much more difficult to bring off the operation success- 
fully. It is at right angles to the plane of the first kind. 
The tetrahedral form of structure is often reproduced 
in the form of the whole diamond, though no one, I 
believe, knows exactly why the faces of the tetrahedron 
are often rounded. This does not mean that the 
layers of the atoms are curved, but simply that they 
lie on one another like a series of steps. A structure 
so tightly bound together is brilliantly clear from the 
optical point of view. 
There is another form of carbon crystal, that of 
graphite or black lead, the properties of which seem so 
different from those of the diamond that it is difficult 
to believe they are of the same element and, moreover, 
of much the same construction. One common feature 
is of great interest, namely, the existence in both cases 
of layers of atoms arranged in hexagonal pattern. It is 
difficult to express in words, but the illustration (Fig. 2) 
will make it clear. Each atom is still bound to three 

Fic. 2.—The fine lines of the diagram show the structure of graphite. By 
moving the top layer to the position shown by the broken lines the 
diamond structure ts obtained. 
of its neighbours by the same strong ties as before, 
but the fourth is broken and a weaker, lengthier 
connexion is substituted. All this is reproduced in _ 
the outward appearance of graphite. Its crystals are 
badly formed, but are more or less in hexagonal 
columns, which split up with the greatest ease into 
thin leaves at right angles to the column axis. So 
easy, indeed, is this cleavage that the pounding of a 
mass of graphite in a mortar is ludicrously ineffective. 
The leaves simply multiply themselves more and more. 
One leaf slides on another very easily, yet the atoms 
in each leaf hold well together. It is the combination 
23 
