THE COEFFICIENT OF SIMPLE RIGIDITY AND 
YOUNG’S MODULUS FOR 
HEXAGONAL CRYSTALS OF SELENIUM 
L. P. SIEG and R. F. MILLER 
Elastic constants determined from drawn wires are of consid- 
erable practical use, but they are not very illuminating on the sub- 
ject of the structure of matter. A drawn wire is a heterogeneous 
mass of matter in crystalline and amorphous states, with never any 
regularity in composition or of crystalline structure. In fact the 
elastic constants so determined are not so much characteristic of 
the substance, say copper, as of the particular physical state of that 
substance. If, however, one could employ an isolated crystal of a 
simple elementary substance, he ought to be able to determine the 
elastic constants of the substance itself, rather than that of its par- 
ticular physical state. 
This thought has been in mind for some time, in fact ever since 
the laboratory has had so liberal a supply of large selenium crys- 
tals which were made by Dr. F. C. Brown, of the Bureau of Stan- 
dards. Last summer (1920) we undertook the determination of 
the simple rigidity and of Young’s modulus for some of these crys- 
tals, and the following account records the results of these first ex- 
periments. 
In figure 22 we have a photograph of some typical crystals with 
which we worked. The length of one cm. is indicated on the pho- 
tograph, so that the actual sizes of the crystals can be estimated 
There were available many larger crystals, but careful selection 
indicated that the smaller ones were more regular. The ideal form 
of these crystals is that of slender regular haxagonal prisms, but 
actually they proved to vary from this in two respects : the forms 
were really those of truncated prisms, and the sections, while hex- 
agonal, were not regular. The angles, as is generally true in crys- 
tals were practically perfect, in this case 120°, but the sides varied 
in length. These two irregularities were fortunately slight, for oth- 
erwise anything like accuracy in determining the elastic constants 
would be out of the question. 
