COAL-TAR AISTD 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 (4^) 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 diflerent 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 Fm=( v^)\ ^^c^~^)' where Fm = viscosity 



of the solution, Ft = viscosity of the tar, Fc = 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 Fc= (0.31''-') (0.025<'-^). 



