PHYSICS: A. W. HULL 
471 
of which aluminum, copper, silver, and probably gold and lead are 
examples, where each atom has twelve equidistant nearest neighbors; 
and the closest-packed hexagonal arrangement described below, of 
which magnesium is at present the only example, where the number of 
equidistant nearest neighbors is also twelve, but in a slightly different 
arrangement from those of the face-centered cubic lattice. 
The X-ray analysis was made in two parts. First, single small 
crystals were mounted with definite orientation on the spectrometer 
table, and photographed while slowly rotated and exposed to a mono- 
chromatic beam of X-rays. This gave the approximate structure. A 
picture was then taken of magnesium powder, in the manner described 
in a previous paper {Phys. Rev. 9, 85, Jan. 1917), which checked and 
confirmed the results of the first method. 
Three small samples, formed by vacuum distillation, were used for the 
single photographs. The first was mounted with its basal plane (0001) 
parallel to the rays, and was rotated about the axis (0001) - (lOTO) for 
about 30° on each side of the center. This should give reflection from 
(0001) and the flatter pyramids (1013), (10T2), (lOTl), etc. The second 
was mounted so as to rotate about the same axis, but with lOlO parallel 
to the rays at the center position. This should give reflection from 
(lOTO) and the steeper pyramids (3031), (2021), (1011), etc. The 
third was rotated about the axis (0001), (1120), with rays parallel to 
1120 at center, so as to give reflection from (1120), (1121), (1122), etc. 
The observed lines and spacings are given in table 1. 
TABLE 1 
CRYSTAL 1 
CRYSTAL 2 
CRYSTAL 3 
Position 
of line 
Spacing 
of plane 
Plane 
Position of 
line 
Spacing of 
plane 
Plane 
Position of 
line 
Spacing of 
plane 
Plane 
3.10 
2.90 
3.30 
4.20 
5.50 
6.27 
2.59 
2.75 
2.44 
1.90 
1.48 
1.30 
0001 
lOTO 
lOTl 
10T2 
10T3 
2051 
2.90 
3.10 
3.30 
6.05 
8.92 
5.9 
2.75 
2.59 
2.44 
1.34 
0.92 
1.38 
lOTO 
0001 
lOTl 
2051 
10T3(3) 
10T0(2) 
5.02 
5.50 
6.0 
1.60 
1.48 
1.36 
1150 
1151 
1152 
The first column gives for each crystal the distance of the observed fine 
from the center, the second the spacing of the corresponding plane, as 
calculated from this distance, and the third the indices of the plane. 
A triangular prism having the spacing = 1.61 A and axial ratio 
1.624 would have a height 1.624 X 2^1120 = 5.23 A, which is exactly 
twice the spacing of the (0001) planes found above, and suggests that 
the lattice is composed of two sets of triangular prisms each of side 3.22 A 
