78 BULLETIN 1036, TJ. S. DEPARTMENT OF AGRICULTURE. 
the tar-acid and tar-base contents, the determination of the ratio of 
the specific gravities to the indices of refraction values of the various 
fractions, and the testing of the toxicity of the creosotes. The 
absence of tar acids or tar bases is proof that the oil under test is not 
a distillate of coal tar. Part of the tar acids or tar bases may 
be removed from coal-tar creosote by chemical methods, but it is 
not likely that all the tar acids and all the tar bases would be taken 
out by such means. On the other hand, the presence of tar acids or 
tar bases or both classes of compounds does not mean that the oil 
under examination is a pure coal-tar distillate. It may be a water- 
gas-tar oil which has been mixed with a coal-tar creosote containing 
large amounts of tar acids and bases, or these materials may have 
been added directly to water-gas-tar creosote. The determination of 
the ratio of the specific gravities of the fractions to their index of 
refraction values is of use only if it is known that the distillates are 
pure oils, either water-gas-tar or coal-tar distillates. Unknown 
mixtures of the two can not be differentiated with certainty by this 
method. The toxicity test is of value only in high-boiling oils similar 
to carbolineum. In lower-boiling oils the fraction similar to car- 
bolineum might be tested, but because of the fact that six weeks are 
required for its completion this test is of little value for commercial 
purposes. 
