SAMPLING AND TESTING HIGHWAY MATERIALS. 
37 
This pycnometer consists of a conical or Erlenmeyer-shaped flask about 4.5 cm. 
high, 4.0 cm. diameter at bottom, and 2.5 cm. diameter at the mouth. It is care- 
fully ground to receive an accurately fitting solid glass stopper with a hole about 1 mm. 
bore in place of the usual capillary opening. The lower surface of this stopper is made 
concave to allow air bubbles to escape through the bore. The depth of the cup-shaped 
depression is 4.8 mm. at the center. The flask has a capacity of about 25 c. c. and 
weighs when empty about 25 grams. Its principal advantages are (1) that any de- 
sired amount of bitumen may be poured in without touching the sides above the de- 
sired level; (2) it is easily cleaned; (3) on account of the 1.0 mm. bore the stopper 
can be easily inserted when the flask is rilled with a viscous oil. 
When working with semisolid -bitumens which 
are too soft to be broken and handled in fragments, 
the following method of determining their specific 
gravity is employed : 
The clean, dry pycnometer is first weighed 
empty, and this weight is called a. It is then 
filled in the usual manner with freshly distilled 
water at 25° C, and the weight is again taken and 
called b. A small amount of the bitumen should 
be placed in the spoon and brought to a fluid 
condition by the gentle application of heat, with 
care that no loss by evaporation occurs. When 
sufficiently fluid, enough is poured into the dry 
pycnometer, which may also be warmed, to fill it 
about half full without allowing the material to 
touch the sides of the tube above the desired 
level. The flask and contents are then allowed to 
cool to room temperature, after which the tube is 
carefully weighed with the stopper. This weight 
is called c. Distilled water, at 25° C, is then 
poured in until the pycnometer is full. After 
this the stopper is inserted, and the whole cooled 
to 25° C. by a 30-minute immersion in a beaker of 
distilled water maintained at this temperature. All f 
surplus moisture is then removed with a soft cloth, J 2^/7/0. 
and the pycnometer and contents are weighed. 
This weight is called d. From the weights ob- 
tained the specific gravity of the bitumen may be 
readily calculated by the following formula: 
(b-a)-(d-c) 
/cm. 
Specific gravity 25° C./25° C.= 
c-a 
Fig. 1G.— Hubbard-Carmick pycnometer. 
(Dimensions only approximate.) 
Both a and b are constants and need be deter- 
mined but once. It is therefore necessary to make 
but two weighings for each determination after the first. Results obtained according 
to the method given above are accurate to within 2 units in the third decimal place, 
while the open-tube method is accurate to the second decimal place only. 
The specific gravity of fluid bitumens may be determined in the ordinary manner 
with this pycnometer by completely filling it with the material and dividing the 
weight of the bitumen thus obtained by that of the same volume of water. 
The pycnometer may be readily cleaned by placing it in a hot-air bath until the 
bitumen is sufficiently fluid to pour. As much is drained out as possible and the 
interior swabbed with a piece of cotton waste. It is then rinsed clean with a little 
carbon disulphide, and after drying is again ready for use. 
