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BULLETIN 314, X'. S. DEPARTMENT OP AGRICULTURE, 



average pressure for a given laboratory, the variations in results due 

 to varying pressures when the thermometer is afterwards used for 

 distillation will be no greater than the possible errors in distillation. 

 The method as a whole is practically the same as that tentatively 

 recommended in 1911 by the Committee on Standard Tests for 

 Road Materials of the American Society for Testing Materials. 1 

 Briefly described, it consists in distilling 100 

 cubic centimeters of the refined or dehydrated 

 tar in an Engler flask at a uniform rate of 1 

 cubic centimeter per minute and collecting the 

 various fractions in weighed glass graduates. 

 In preparing for the test it will be found con- 

 venient to mark permanently on the foot of 

 each graduate its weight to within 0.1 gram. 

 The flask should be supported in a vertical 

 position on one pan of the rough balance and 

 its tare accurately obtained. From the specific 

 gravity of the tar, the weight of 100 cubic cen- 

 timeters is calculated, and this amount, after 

 warming it in a tin cup, if necessary to make 

 it sufficiently fluid, is poured into the tared 

 flask. A cork stopper carrying the thermome- 

 ter is then inserted in the neck of the flask, 

 so that the top of the 

 bulb is opposite the mid- 

 dle of the tubulature, 

 and the entire appara- 

 tus set up as shown in 

 figure 12. A tin shield 

 with small sight hole 

 surrounds the flask and 

 burner as shown in order 

 to obviate the influence 

 of drafts. 



The tar should be 

 heated gradually by 

 means of a Bunsen burner, and the heat should be so regulated as to 

 maintain distillation at the constant rate of 1 cubic centimeter per 

 minute. When the thermometer registers a temperature correspond- 

 ing to 110° C, the graduated cylinder containing the first fraction is 

 replaced by another. The receiver is changed again at 170° C. and at 

 270° C, using as many graduated cylinders as maybe necessary with- 

 out allowing any to become filled above the 25-cubic-centimeter mark. 



Fig. 12. — Distillation apparatus 



i Proc. Am. Soc. for Testing Materials, 1911, Vol. XI, p. 240. 



