66 AN INVESTIGATION INTO 
Determinations of the cubic compressibility of glass, D, have been made by 
other observers using various methods. The results go to show that different 
varieties of glass vary considerably in their compressibility. These determi- 
nations may be tabulated as follows :* 
Everett 5,074,600 to 6,379,400 (C. G. S.= 3-5 to 4.4 x io n ). 
Amagat Common glass 6,745,000 (.000002181 per atmosphere). 
Amagat Crystal glass 6,112,300 (.000002405 per atmosphere). 
Tait 5.657,700 (.0000026 per atmosphere). 
As will be seen, the figures obtained for plate glass in the present investiga- 
tion lie a little above the average of the various values here given, and are 
nearly those of the highest value obtained by Everett. 
SUMMARY OF RESULTS. 
The table on page 69 gives a summary of the average values obtained 
for E, ff, C and D in the case of all rocks examined in this investigation. 
With these are placed, for purposes of comparison, the results obtained 
for these constants in the case of wrought iron, cast iron and glass. In the 
second table on page 69 these values are again presented, recalculated into 
C. G. S. units. 
The rocks fall naturally into three groups, differing from one another in 
compressibility, but the several members of each group agreeing fairly closely 
among themselves. 
These three groups show a corresponding difference in composition. 
The first group consists of the marbles and limestones. These have an aver- 
age value for D of 6,345,000. One of these, however, the Black Belgian marble 
which is very much finer in grain than the others and breaks almost like a piece 
of glass, has a very much higher value for D than that possessed by the other 
rocks which among themselves are nearly identical. If we omit this Belgian 
marble, the average of D for the other limestones and marbles, is 5,855,000. 
The second group comprises the granites. These again show a close agree- 
ment of values among themselves, except in the case of the Stanstead granite* 
which rock, as already mentioned, shows a defective elasticity. The average 
value of D for the granites is 4,399,000. 
The third group embraces the basic intrusives (gabbro, anorthosite, essexite, 
and diabase). These show greater differences, but have an average value 
for D of 8,825,000. The nepheline syenite, although higher in silica and 
therefore properly speaking an acid rock ; in its freedom from quartz, and its 
richness in feldspar (although the feldspar is largely orthoclase instead of 
plagioclase), in mineralogical composition belongs with these basic rocks 
rather than with the granites. It also approaches the essexite most nearly in its 
compressibility. 
*See Everett, Illustrations of the C. G. S. System of Units with tables of Physical Con- 
stants. MacMillan&Co., 1902, pp. 60 to 64. The figures there expressed in various units 
have been here recalculated into inch-pound values. 
