COAL-TAR AND WATER-GAS TAR CREOSOTES. 
83 
loss is based on an increased weight resulting from the addition of 
tar. The same general conclusion that the volatility of the creosote 
itself is practically not affected by the addition of the tar has been 
reached by Fredendoll (0) with respect to high-boiling petroleum 
oils. 
VISCOSITY. 
The addition of tar to coal-tar creosote of course increases the 
viscosity. The increase is not, however, in direct proportion to the 
amount of tar added. Up to about 50 per cent of tar the increase 
is relatively small, but beyond that point the viscosity increases very 
rapidly. Figure 39 shows the effect upon the viscosity of adding 
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Fig. 39..— Change in viscosity of tar solutions 
with different amounts of tar at various 
temperatures. 
1— At 40° C 2— At 50° C 3— At 70° C 
4— At 90° C 
tar to creosote at different temperatures. The viscosity of any solu- 
tion of coal tar and creosote may be calculated from the viscosity 
of the tar and the viscosity of the creosote at the same temperature 
by the aid of the formula V m =l y— )( T 7 C 1_P j> where F m = viscosity 
of the solution, V t = viscosity of the tar, V c = viscosity of the creosote, 
P = percentage tar used expressed as a decimal. 
In figure 39 the value of a 50 per cent solution of tar and creosote 
at 90° C. is obtained from the equation F c = (0.31 - 5 ) (0.025 05 ). 
