472 
PHYSICS: A. W, HULL 
and height 5.23A, the atoms of either set being in the center of the 
prisms of the other. The number n of atoms per unit prism is, if p is the 
density, M the mass of an atom of Mg, and a and h represent the side 
and height of the prism, respectively: 
which is equal to 1 within the limit of accuracy of a, h, and p, and is 
correct for the assumed structure. 
The second step in the analysis was to check this assumed structure by 
a photograph taken through finely powdered magnesium, which should 
show all the lines required by the assumed structure and no more. Table 
2 gives the position of the observed lines and the corresponding spacing 
TABLE 2 
spacing OF PLANE IN ANGSTROMS 
DISTANCE OF LINE 
ANGLE OF 
INTENSITY 
INDICES OF 
FROM CENTER 
REFLECTION 
(estimated) 
PLANE 
Experimental 
Theoretical 
7 40° 
70 
/ U . 
2.75 
2.75 
lOTO 
2.59 
0001 
8 33 
2.44 
2.44 
inTi 
4.23 
10.74 
50.0 
1.91 
1.90 
10T2 
5.03 
12.75 
70.0 
1.61 
1.60 
1120 
5.50 
13.90 
60.0 
1.48 
1.48 
10T3 
1.38 
lOTO (2) 
5.95 
15.00 
60.0 
1.36 
1.36 
1122 
6.05 
15.30 
20.0 
1.34 
1.34 
2021 
1.30 
0001 (2) 
6.65 
16.80 
10.0 
1.23 
1.23 
lOTl (2) 
6.90 
17.40 
0.5 
1.18 
1.18 
1014 
7.52 
19.00 
15.0 
1.09 
1.08 
2023 
7.96 
20.10 
30.0 
1.04 
1.05 
2130 
8.10 
20.50 
0.5 
1.02 
1.03 
2131 
1.01 
1124 
8.41 
21.3 
20.0 
0.98 
0.97 
2132 
10T5 
0.94 
10T2 (2) 
8.83 
22.4 
0.3 
0.93 
0.92 
lOTO (3) 
9.15 
23.1 
15.0 
0.90 
0.89 
2133 
9.48 
24.0 
10.0 
0.87 
0.87 
3032 
0001 (3) 
9.90 
25.1 
0.5 
0.83 
0.83 
10T6 
2025 
0.82 
lOTl (3) 
2134 
0.80 
1120 (2) 
10.95 
27.7 
10. 0 
0.77 
0.77 
3140 
