“EXAMINATION OF BITUMINOUS. ROAD MATERIALS, 5 
the hydremeter sinks slowly, owing to the viscosity of the material, 
it should be given sufficient time to come to a definite resting point, 
and this point should be checked by raising the hydrometer and allow- 
ing it to snk a second time. The hydrometer should never be pushed 
below the point at which it naturally comes to rest until the last 
reading has been made. It may then be pushed below the reading 
for a distance of three or four of the small divisions on the scale, 
whereupon it should immediately begin to rise. If it fails to do so, 
the material is too viscous for the hydrometer method, and the 
pycnometer method should be employed. 
The direct specific gravity reading obtained by the foregoing 
method is based upon water at 15.5° C. taken as unity. For all 
practical purposes this reading may be corrected to water at 25° C., 
considered as unity, by multiplying it by 1.002. Thus: 
Specie gravity 25° C./25° C. specific gravity 25° C./15.5° C. x 1.002 
PYCNOMETER METHOD (USED FOR VISCOUS FLUID AND SEMISOLID BITUMENS AND 
EMULSIONS). 
EQUIPMENT. 
1 large metal kitchen spoon. 
1 steel spatula or kitchen knife. 
1 Bunsen burner and rubber tubing. 
1 250 cubic centimeter low-form glass beaker. 
1 chemical thermometer reading from —10° C. to 110° C. 
1 special pycnometer. (Fig. 2.) 
1 analytical balance, capacity 100 grams, sensitive to 0.1 milligram. 
METHOD. 
The inconvenience and difficulty of employing the ordinary narrow- 
neck pycnometer when determining the specific gravity of viscous 
fluid and semisolid bitumens has led to the use of the special form 
shown in figure 2. 
This pycnometer consists of a fairly heavy, straight-walled glass 
tube, 70 millimeters long and 22 millimeters in diameter, carefully 
ground to receive an accurately fitting solid glass stopper with a hole 
of 1.6 millimeters bore in place of the usual capillary opening. The 
lower part of this stopper is made concave in order to allow ail air 
bubbles to escape through the bore. The depth of the cup-shaped 
depression is 4.8 millimeters at the center. The stoppered tube has a 
capacity of about 24 cubic centimeters and when empty weighs about 
28 grams. Its principal advantages are (1) that any desired amount 
of bitumen may be poured in without touching the sides above the 
level desired; (2) it is easily cleaned; (3) on account of the 1.6-milh- 
meter bore the stopper can be more easily inserted when the tube is 
filled with a very viscous oil than if it contained a capillary opening. 
