62 
AN INVESTIGATION INTO 
The stress-strain curves are shown in figure 24. 
As will be seen, the rock displays a marked hysteresis and is not there- 
fore an ideal material for the application of this method of determining com- 
pressibility. 
The results obtained are as follows : 
E= 2,290,000; 
0.29; D= i, 816,000; C= 888, ooo. 
4000 
80 120 160 200 24O 280 320 
STRAIN 
FIG. 24. Sandstone. Stress-strain curves. 
360 
4OO 44O 
THE ELASTIC CONSTANTS OF GLASS. 
As in geophysical speculations, the earth in respect to its rigidity and com- 
pressibility is often compared to a globe of glass, it seemed advisable to deter- 
mine as accurately as possible the elastic constants of glass, for the purpose of 
comparing them with the results obtained in the case of the various rocks 
considered in this paper, employing the same methods and carrying out the 
work under exactly the same conditions. This material lends itself excellently 
to this method of measuring these constants, provided the glass is free from 
all irregularities in its substance and is iso tropic in character. The first diffi- 
culty experienced was that of obtaining such a glass. At the outset it was 
thought that thick glass rods such as are used for various purposes in the 
chemical and physical laboratory might be employed, but although several 
lots of the purest variety of this material were procured, the glass constituting 
it was found in all cases to contain minute air bubbles, and when examined 
between crossed nicols in polarized light, showed brilliant colors red, 
yellow, and blue. This indicated a state of marked tension in the glass, evi- 
dently due to the rod having been drawn when the glass was in a viscous state, 
which was also shown by the circular arrangement of the little bubbles in the 
rod, following the direction of its surface. Short lengths of this rod, moreover, 
when tested in compression, so soon as the maximum load had been exceeded, 
instead of splitting from top to bottom, broke as if composed of a series of 
rudely concentric shells. All attempts on the part of the various glass makers 
to whom this glass was submitted for a thorough annealing, failed to remove 
or in fact to reduce to any considerable extent this aniso tropic condition.. 
