22 BULLETIN 314, U. S. DEPARTMENT OF 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.' 
Briefly ceseribed, 1t consists in distilling 100 
cubic centimeters of the refined or dehydrated . 
tar in an Engler fiask 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 cf 
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 tareaccurately obtained. From the specific 
eravity 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 may be necessary with- 
out allowing any to become filled above the 25-cubic-centimeter mark. 
FIG. 12.—Distillation apparatus. 
1 Proc. Am. Soc. for Testing Materials, 1911, Vol. XI, p, 240. 
