6 



BULLETIN 314, U. S. DEPARTMENT OP AGRICULTURE. 



When working with semisolid bitumens which are too soft to be 

 broken and- handled in fragments, the following method of deter- 

 mining their specific gravity is employed, The clean, dry pycnom- 

 eter is first weighed empty and this weight is called r< a." It is 

 then filled in the usual manner with freshly distilled water at 25° C, 

 and the weight is again taken and called f '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 tube 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 surplus 

 moisture is then removed with a soft cloth, and the pyc- 

 nometer and contents are weighed. This weight is called 

 "d." From the v/eights obtained the specific gravity of 

 the bitumen may be readily calculated by the following 

 formula : 



sa 



Specific gravity 25° C./25 C. = 



C-£ 



(b-a) — (d-c) 



fig.2.— pyc- Both "a" and "b" are constants and need be deter- 

 (Hubbara mined but once. It is therefore necessary to make but 

 type). t wo W eighings for each determination after the first. Re- 



sults 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. 



