PHYSICAL PROPERTIES OF ROAD MATERIALS. 11 
with, an impact machine constructed on the principle of a pile driver. 
The blow is delivered by a hammer weighing 2 kilograms, raised by a 
sprocket chain and released automatically by a concentric electro- 
magnet to fall on an armor-piercing steel plunger with spherical 
lower end which is in contact with the upper surface of the test piece. 
The test consists of a 1 centimeter fall of the hammer for the first 
blow and an increased fall of 1 centimeter for each successive blow 
until failure of the test piece occurs. The number of blows required 
to cause rupture represents the toughness. 
The cementing value is the property possessed by a rock dust to 
cement or bind together the coarser rock fragments, and the test is 
carried out as follows : One-half kilogram of rock is broken sufficiently 
small to pass a half -inch-mesh, screen and is then moistened with 90 
cubic centimeters of water and placed in a cast-iron ball mill, contain- 
ing two cast-steel shot, 5 inches in diameter and weighing about 20' 
pounds each. The sample is ground for 2\ hours at the rate of 2,000' 
revolutions per hour or until the material has been reduced to a thick 
dough, the particles of which are not above 0.25 millimeter in diame- 
ter. The dough is then removed and molded into cylindrical bri- 
quettes 25 millimeters in diameter and 25 millimeters high, in a 
hydraulic briquette-forming machine, so adjusted as to give a maxi- 
mum momentary pressure of 132 kilos per square centimeter on the 
compressed material. Five briquettes are made from each test 
sample which, after being thoroughly dried at 200° F. and cooled in 
a desiccator, are broken by an especially designed impact machine 
having a 1 -kilogram pendulum hammer with an effective drop of 
1 centimeter. The average number of blows required to destroy the 
bond of cementation in five briquettes determines the cementing 
value of the rock sample. 
Specific gravity is the weight of the material compared with that 
of an equal volume of water, and is obtained by dividing the weight 
in air of a rock fragment by the difference between its weight in air 
and water. The weight per cubic foot of rock is found by multiplying 
the specific gravity by 62.37 pounds, the weight of a cubic foot of 
water. 
GENERAL RELATION BETWEEN THE PHYSICAL PROPERTIES OF ROCKS 
AND THEIR MINERAL COMPOSITION AND STRUCTURE. 
The general relation of rocks as expressed by average mineral 
composition and physical properties may be seen from the results of 
the tests and analyses given in Table 1. In the first place it will be 
noticed that igneous rocks as a class (Nos. 1-14), owing to their more 
uniform structure and preponderance of hard silicate components, 
have better indicated wearing properties than either sedimentary 
rocks or crystalline schists. As previously stated, these igneous rocks 
