TESTS OF GRANITE. 65 
volume calculated. He found, assigning to emery an arbitrary value 
of 1,000 as representing its average hardness, that granite from 9 
localities showed the following degrees of hardness: 31.7, 38.1, 41.7, 
44.8, 48.4, 50.7, 52.9, 56.6, and 67.1. The extremes of these figures 
show that some granites have a general hardness more than twice as 
great as others. 
J. F. Williams " proposed to determine the relative hardness of 
granites by noting the rate of penetration of a drill of a given 
diameter, or by measuring the distance to which such a drill will 
penetrate without being sharpened, or the amount of surface of 
rough-pointed granite which can be reduced to a bush-hammered 
surface per hour. Since the introduction of pneumatic drills and sur- 
facers these methods can be easily applied. 
Compressive strength. — The methods of testing the strength of 
building stones have grown in precision. The first requisite is that 
the cubes to be tested should be sawed by diamond sa \vs and not ham- 
mered out. The next is that the direction of both rift and grain 
should be indicated thereon, and that three cubes should be tested, one 
with pressure applied parallel to the direction of the rift, one applied 
parallel to that of the grain, and the third at right angles to rift 
and grain. Where the rift and grain are pronounced the three 
results will differ. As in the reports of tests made with the testing 
machine at the Watertown Arsenal, Mass., the number of pounds 
pressure at which the first crack is produced should always be 
given, as well as that at which the cube is crushed. It is assumed 
that these tests are made in a dry atmosphere. 
Transverse strength, shearing strength, and compressive elasticity. — 
It has been found useful for certain architectural purposes to test 
these qualities in granite. ^ 
Porosity. — Buckley points out ° that the danger from frost depends 
not upon the amount of absorption but upon the size of the pore 
space. Rocks with large pore spaces stand frost better than those 
with small ones, because they do not retain the water that they absorb. 
Tests of porosity are therefore important. Buckley used the dry and 
saturated weights obtained for the samples used in computing the 
specific gravity. 
The difference in these weights was multiplied by the specific gravity of the 
rock. This amount was added to the dry weight, giving the sum. The differ- 
ence of the dry and saturated weights multiplied by the specific gravity of the 
rock was then divided by the sum. This last result is the actual percentage of 
pore space compared with the volume of the sample tested. 
a Igneous rocks of Arkansas : Ann. Rept. Geol. Survey Arkansas, vol. 1, 1890, p. 41. 
6 See Buckley, Building and Ornamental Stones of Wisconsin, pp. 396-398. Also 
Rept. of tests of metal, etc., Watertown Arsenal (1895), 1896, pp. 319-322, 339-351, 
407-411. Some of the results as to elasticity are given on page 21 of this report. 
c Buckley, op. cit, pp. 68, 69, 372-376, 400, 413. 
3495— Bull. 313—07 5 
