104 
IOWA ACADEMY OF SCIENCE Vor.. XXVIII, 1921 
Fig. 22. Types of selenium crystals used in the determination of their elastic con- 
stants. 
I. YOUNG’S MODUEUS 
For the determination of Young’s modulus, the method of bend- 
ing was used. The crystal studied was carefully measured, then 
laid on two parallel knife edges at a known distance apart, and 
the bend at the middle, for various loads, was recorded. The bends 
were observed with a traveling rriicroscope. In fact everything 
proceeded as in an ordinary laboratory experiment for this deter- 
mination, excepting that everything was on a smaller scale, and 
correspondingly smaller distances had to be measured. A sche- 
matic drawing of the arrangement of crystals and apparatus is 
shown in figure 23. The microscope was used to measure the 
Fig. 23. Arrangement of apparatus for determining the bending of crystals 
