A Contribution to 'the Study of the Diamond Made. 
167 
intricate grouping of atoms than that of the " particles " shown in Fig. 7. 
He finds a primitive cube, and draws it so that the points C, D, H, Gr, of 
Fig. 7 would be the mid points of its edges, and then deduces atoms 
correspondingly with a, h, c, d, e, f, g, h, h, I, m, n, but finds another atom 
point at the mid point of PQ, together with four others asymmetrically 
placed. To quote his own words : " When all the information is put 
together we find that the element of volume of the diamond is a face- 
centred cube ; a cube having, that is to say, a carbon atom at each corner 
and one in the middle of each face. In the same cube are also four carbon 
atoms at the centres of four of the eight small cubes into which the large 
cube may be divided." In other words his cube coincides with mine 
excepting that its outline is shifted aside by half an edge, and that it 
contains five extra atoms which are not represented by particles in my 
drawing, and which I have been unable to derive The spacing between the 
planes of atoms parallel to the faces of the cube (100), the dodecahedron 
(110) and the octahedron (111) is the same as for the " particles," namely 
as 1 : t/2 : \/3, whether the five extra atoms are included or not. 
By placing a particle at the origin (0) of the six-rayed figure, i. e. at the 
centre of the cube in Fig. 7, we should have 
dO : dbidg = pI^2 : p : \/3p/\/2 
= 1 : \/2 : n/b: 
In this case successive ray-spaces might be placed face to face, whence the 
juxtaposition of adjacent halves of the ray-spaces would give a true lattice 
of face-centred cubes. The outside halves, however, would be derelict and 
the development of the octahedron not easily imaginable. 
Eutherford seems to have had some difficulty Avith the structure found 
by Bragg, for in describing it (' Ann. Rep. Smithsonian Inst.,' 1915) he 
calls it cubical but complicated, and the " atoms are all equidistant, but the 
general arrangement differs markedly from that of rock salt. It is seen that 
each carbon atom is linked with four neighbours in a perfectly symmetrical 
way, while the linking of six carbon atoms in a ring is also obvious from the 
figure. The distance between the plates containing atoms is seen to 
alternate in the ratio 1:3." But neither this account nor the picture of the 
model made to illustrate it seems quite to agree with what Bragg said. 
