54 DEPARTMENT BULLETIN 12 10, U. S. DEPT. OF AGRICULTURE 
the pan and weighed. This weight is called a. It is then weighed immersed 
in water at 25° C and this weight is called b. The specific gravity may then be 
calculated by means of the following formula: 
Specific gravity = _, 
DISPLACEMENT METHOD 
(Used for semisolids) 
11. The specific gravity of semisolid bituminous materials may be determined 
by the displacement method. Weigh a silica crucible suspended from the beam 
of the balance in air and call the weight a, and in water and call the weight b. 
Fill the crucible approximately two-thirds full with the material under examina- 
tion. Free from bubbles by heating on a hot plate, cool ami weigh, calling this 
weight c. Immerse the filled crucible in water at 25° C. for one-half hour, 
then suspend by a wire from the beam of the balance and weigh it immersed in 
water at 25° C. : call this weight d. The specific gravity is then calculated by 
means of the following formula : 
o. -r* • c — a 
Specific gravity = ^37? 
44. METHOD OF TEST FOR PERCENTAGE OF BITUMEN 
(SOLUBLE IN CARBON DISULPHIDE) 
1. This test consists in dissolving the bitumen in carbon disulphide and recov- 
ering any insoluble matter by filtering the solution through an asbestos felt. 
The form of Gooch crucible best adapted for the determination is 4.4 centimeters 
wide at the top, tapering to 3.6 centimeters at the bottom, and is 2.5 centimeters 
deep. 
2. For preparing the felt, the asbestos is cut with scissors into pieces not ex- 
ceeding 1 centimeter in length, after which it is shaken up with just, sufficient 
water to pour easily. The Gooch crucible is filled with the suspended asbestos, 
which is allowed to settle for a few moments. A light suction is then applied 
to draw off all the water and leave a firm mat of asbestos in the crucible. More 
of the susj tended material is added, and the operation is repeated until the felt 
is so dense that it scarcely transmits light when held so that the bottom of the 
crucible is between the eye and the source of light. The felt should then be 
washed several times with water and drawn firmly against, the bottom of the 
crucible by an increased suction. The crucible is removed to a drying oven for a 
few minutes, after which it is ignited at red heat over a Bunsen burner, cooled 
in a desiccator, and weighed. 
3. From 1 to 2 grams of bitumen or about 10 grains of an asphalt topping or 
rock asphalt are now placed in the Erlenmeyer flask, which has been previously 
weighed, and the accurate weight of the sample is obtained. One hundred cubic 
centimeters of chemically pure carbon disulphide are poured into the flask in 
small portions, with continual agitation, until all lumps disappear and nothing 
adheres to the bottom. The flask is then corked and set aside for 15 minutes. 
4. After being weighed, the Gooch crucible containing the felt is set up over 
the dry-suction flask, and the solution of bitumen in carbon disulphide is de- 
canted through the felt with light suction or without suction by gradually tilt- 
ing the flask, with care not to stir up any precipitate that may have settled out. 
At the first sign of any sediment coming out, the decantation is stopped and the 
filter allowed to drain. A small amount of carbon disulphide is then washed 
down Hie sides of the flask, after which the precipitate is brought upon the felt 
and the flask scrubbed, if necessary, with a feather or "policeman" to remove 
all adhering material. The contents of the crucible are w r ashed with carbon 
disulphide. until the washings run colorless. Suction is then applied until there 
is practically no odor of carbon disulphide in the crucible, after wmich the out- 
side of the crucible is cleaned with a cloth moistened with a small amount of 
solvent. The crucible and contents are allowed to dry over the top of the oven 
until no odor of carbon disulphide is detected and then dried in the hot-air oven 
at 100° C. for about 20 minutes, cooled in a desiccator, and weighed. If any 
