no 
IOWA ACADEMY OF SCIENCE Vol. XXVIII, 1921 
For a regular hexagon, / = a^, and w = in terms 
o Z 
of the length of side, a. Then -- = .0237 
notation as in the first section, 
Using the same 
therefore 
and (13) becomes 
a-c + j- 
.(14) 
J Q 
Finally we have the expression for n, by combining (12) and 
(14) 
1 AH ^2 7^. r r nn.7Q 'i r i i ^ 
.(IS) 
mir^IoL (.0079) r 1 
r2 [ e j[c^ 
1 
,C3~ (C + ^)3j 
Four crystals were used in this determination, the data and re- 
sults for which are given in Table IV. 
table IV 
Crystal 
Sides, in micra. 
, taken in order 
Small End 
Large End 
2 
121, 77, 114, 115, 81, 103 
141, 134, 131, 154, 132, 147 
3 
128, 142, 123, 115, 144, 105 
164, 186, 146, 167, 173, 132 
11 
65, 29, 62, 67, 24, 69 
84, 43, 78, 77, 42, 74 
12 
81, 76, 35, 87, 78, 44 
93, 128, 73, 115, 111, 99 
Crystal 
r(cm) 
^(cm) 
L(cm) 
rCsec) 
n X 10~i^ 
2 
.0102 
.0038 
0.98 
6.37 
0.664 
3 
.0126 
.0035 
1.66 
7.285 
0.421 
11 
.00527 
.00136 
0.92 
23.46 
0.756 
12 
.00669 
.0036 
1.28 
14.40 
0.750 
Mean 0.65 ±2 -05 
The variation in n is about of the order of the variation in Y. 
Comparing results we have for the mean value of Y, 5.40 X lO^b 
and for n, 0.65 X 10^^ dynes per cm^. Their ratio, Y/n is equal 
to 8.3. This is unusually large, at least compared to isotropic sub- 
stances in which this ratio for common metals averages about 2.6. 
However, we are here dealing with a crystalline substance, not an 
isotropic one, and comparisons with the latter substances are of 
little value. 
It has been shown in crystal theory that to determine completely 
the elastic constants of an hexagonal crystal would require the de- 
