

EXAMINATION OF-. BITUMINOUS EOAD MATERIALS. 5 



the hydrometer 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 sink 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: 



Specific 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 Eunsen 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. 



The inconvenience and difficulty of employing the ordinary narrow- 

 nock 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 .0 millimeters bore in place of the usual capillary opening. The 

 lower part of this stopper is made concave in order to allow all 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 

 /nuns. lis principal advantages are (1) that any desired amount 

 of bitumen may be poured in without touching the sides above the 

 level di ired; (2) it is easily cleaned; (3) on account of the 1. 6-milli- 

 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. 



