6 4 
AN INVESTIGATION INTO THE 
After a prolonged search for iso tropic glass in masses of sufficient size to 
measure the elastic constants, it was found that plate glass answered the 
requirements. A piece of one-inch plate glass made in Great Britain was 
accordingly secured and was cut into strips an inch wide, and these again into 
three-inch lengths. The square prisms thus produced were then properly faced 
and polished. The glass was found to be absolutely free from all flaws and 
impurities and when examined between crossed nicols the prisms, although an 
inch thick, showed in one direction at right angles to vertical axis absolute 
blackness throughout a complete revolution, while in the other direction at 
right angles to this there was during a revolution an alternation of blackness 
with a pale grayish illumination. This change was so slight that, considering 
the thickness of the glass and the sensitiveness of the test, the material may 
9000 
IOOO 
40 80 120 160 200 24O 
STRAIN 
FIG. 26. Plate Glass. Stress-strain curves. 
be considered to be practically free from internal tension and to be iso tropic 
in character. 
In order to get a good average and to eliminate chance errors as far as possi- 
ble, seven of these prisms were taken, and two complete sets of determinations 
were made on each of them, using in every case different pairs of faces. Four- 
teen determinations were thus made of each of the elastic constants. The 
figures obtained are set forth in the table on page 65. 
In this table a complete series of values obtained from each specimen are 
given in double rows. When the average of all these results is taken, the 
values obtained for the several constants of plate glass are as follows : 
=10,500,000; ^=0.2273; D= 6,448,000; C=4,290,ooo. 
The stress-strain curves given by one of the prisms is shown in figure 26. 
In this figure I represents longitudinal compression and II lateral extension. 
