108 
IOWA ACADEMY OF SCIENCE Voi.. XXVIII, 1921 
Thence substituting in (1), where 
JV = 980 dynes 
L = 0.82 cm 
d — 0.00315 cm 
M = 1.008 X 10—8 cm4 
Y = 3.57 X lO'i^ . dynes per cm^. 
The results for all the crystals, employing this same method, are 
given in Table III. While the range in values for Young’s 
TABLE III 
Crystal 
Young’s Modulus 
X 10-1^ 
(c. g. s.) 
2 
3.57 
2 
3.49 
2 
4.86 
5 
3.28 
6 
4.34 
6 
5.70 
8 
6.60 
9 
6.77 
9 
6.85 
9 
8.49 
Mean 5.40 + 0.4 
modulus is large (the probable error being about 7 per cent of the 
mean), it is not so large as the range in values found from tables 
of results for common substances in the- form of dra^n wires. It 
must be remembered that in expressions (7) and (1) it has been 
assumed that the mean of the six sides of the hexagon at the two 
knife edges, which are the numbers employed for and ag, are 
really the proper numbers to use. Since the hexagons are not 
regular, it is not correct to use the mean, but no other simple 
method seemed available. What one really wants is the side of 
the regular hexagon which possesses the same moment of area as 
the given irregular hexagon. An attempt was made to draw to 
scale some of the sections, and to obtain the moments of area by 
calculating each part separately, but the results as far as obtained 
did not differ very much from the present ones, and so the labor 
involved seemed unjustifiable. The mean value for Y indicates . 
an elasticity roughly about one-third that of steel. 
II. COEFFICIENT OF SIMPLE RIGIDITY 
The experimental determination of the simple rigidity involved 
even more experimental difficulties than those encountered in the 
determination of Young’s modulus. The method of vibration of 
' a torsion pendulum, using the crystal as the support, was em- 
ployed. From a knowledge of the period of vibration, the moment 
