106 
IOWA ACADEMY OF SCIENCE Voi,. XXVIII, 1921 
pan 
460 
800 
1200 
1600 
.426 
.415 
.404 
.391 
.380 
.011 
.011 
.013 
.011 
.293 
.281 
.269 
.256 
.243 
.012 
.012 
.013 
.013 
pan 
.431 
.296 
400 
.419 
.012 
.285 
.011 
800 
.407 
.012 
,271 
.014 
1200 
.395 
.012 
.259 
.012 
1600 
.383 
.012 
.247 
.012 
Mean bend for 400 mg = 0.0121 mm 
Mean bend for 1 gram 0.0303 mm 
Mass of pan = 51 mg 
A summary of all the results for the crystals tested is given dn 
Table 11. 
TABLE II 
Summary of results for all crystals 
Separation of knife edges, 8.2 mm 
Crystal 
Face dimensions, in micra, taken clockwise (as in 
Fig. 24), first face at top 
Bend per 
gram (mm) 
At A 
At B 
2 
121, 77, 114, 115, 81, 103 
136, 120, 127, 144, 119, 136 
.0315 
2 
103, 121, 77, 114, 115, 81 
136, 136, 120, 127, 144, 119 
.0322 
2 
81, 103, 121, 77, 114, 115 
119, 136, 136, 120, 127, 144 
.0231 
5 
94, 36, 54, 79, 40, 40 
109, 20, 74, 83, 47, 42 
.4875 
6 
123, 125, 79, 134, 102, 102 
122, 125, 78, 145, 105, 104 
.0303 
6 
102, 102, 123, 125, 79, 134 
105, 104, 122, 125, 78, 145 
.0231 
8 
41, 48, 47, 38, 58, 48 
62, 66, 77, 40, 85, 53 
.3167 
9 
82, 73, 74, 78, 66, 58 
95, 82, 76, 81, 91, 61 
.0890 
9 
78, 66, 58, 82, 73, 74 
81, 91, 61, 95, 82, 76 
.0880 
9 
58, 82, 73, 74, 78, 66 
61, 95, 82, 76, 81, 91 
0710 
If the crystals had been regular hexagonal prisms one could 
easily have arrived at a value for Young’s modulus by applying the 
formula 
^ AidM 
where Y is Young’s modulus, W the load in dynes, L the length 
between knife edges in cm, and M the moment of inertia (really 
moment of area, for the mass does not enter) of the section about 
the horizontal axis determined by the intersection of the neutral 
plane with the cross section of the crystal. Inasmuch as the 
crystals were not uniform, the exact formula for Y would be well 
nigh impossible of derivation. However, one can use the fol- 
lowing method of approximation with a fair accuracy. In figure 
25 a sketch of a crystal face is given, with a great exaggertion in 
the differences in the two edges, and Og- The former is an edge 
of the section at the knife edge A, and the latter at B. For any 
