COAL-TAR AND WATER-GAS TAR CREOSOTES. 
81 
increase in gravity appears to be a straight-line relation depending 
upon the gravity of the oil, the gravity of the tar, and the proportion 
of each in the mixture. The gravity of any solution may, therefore, 
be calculated from the gravity of its two components by the follow- 
ing formula : Specific gravity solution = per cent tar (specific gravity 
tar) + per cent creosote (specific gravity creosote) . Figure 38 shows 
the change in specific gravity of a creosote with the addition of various 
amounts of a tar. The curve is drawn from the equation : the points 
are from experimental data. 
COEFFICIENT OF EXPANSION. 
No data are available on the coefficient of expansion of tar solu- 
tions; but it would seem probable that the factor could be calculated 
from the known values for 
the change in the specific 
gravities of creosote and tar, 
respectively, on the assump- 
tion that the factor will be 
in direct proportion to the 
percentages of each of the 
two components in the mix- 
ture. J. M. Weiss (15) has 
published the results of in- 
vestigations on the coeffi- 
cient of expansion of tars 
from various sources, and 
shows that the average co- 
efficient of expansion from 
60° F. to 140° F. varies from 
0.00027 to 0.000375 per de- 
gree Fahrenheit, the former 
being for a tar having a specific gravity of 1.296 and th'e latter for a 
tar having a specific gravity of 1.073. If these data be recalculated 
to a change in specific gravity per degree Fahrenheit, the factor will 
vary between 0.00035 and 0.00040, with an average of 0.000375. If, 
then, this factor be taken for the change in specific gravity of tar, 
and the factor 0.00043 be taken for the change in the specific gravity 
of creosote oil, the factors shown in Table 31 will be the change in 
specific gravities of tar mixtures, and should give results close 
enough to the true value for all commercial purposes. 
75535°— 22 6 
118 
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tl6 
114 
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> 
< U ° 
a 
/ 
U 
5 1.08 
hi 
o. 
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1.06 
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KTAG 
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6 
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PERCENTAGE CP10S0TE 
Fig. 38. — The change in specific gravity of tar solution. 
