CHAPTER VI. TAR-CREOSOTE SOLUTIONS. 
Within the last few years tar-creosote solutions have been used 
more and more in wood preservation. Various claims have been 
put forward in favor of tar: First, that, as it is the mother liquor of 
creosote, it contains all the toxic principles of creosote; second, that 
it retards the evaporation of creosote when mixed with it; third, 
that it does not reduce penetration into the wood; and fourth, that 
it is cheaper than coal-tar creosote. In answer to these arguments 
it may be said that, although tar is the mother liquor and contains 
all the toxic principles of coal-tar creosote, the concentration of the 
toxic elements in coal tar is only about one-fourth of that in coal-tar 
creosote, because coal-tar creosote is only about one-fourth of the 
volume of the coal tar from which it is distilled and contains prac- 
tically all the toxic principles. It has also been shown in this bulletin 
that coal tar does not retard the evaporation of creosote mixed with 
it. Furthermore, Bond (17), and Teesdale and MacLean (18) have 
demonstrated that it is more difficult to penetrate wood with coal- 
tar solutions than it is with creosote. 
TAR AS A DILUENT. 
The chief value of tar is as a diluent of creosote, although it may 
have a retarding influence on the rate of solution of the toxic princi- 
ples into the wood. The use of a diluent in wood preservation is 
no new thing. In the treatment of wood with zinc chloride a strength 
of solution is used that will insure a thorough penetration. If the 
wood is difficult to penetrate, a relatively strong solution is used; 
but, if it is easy to penetrate, a more dilute solution is used, and 
more solution is put in, thus insuring a more thorough treatment. 
In other words, the amount of water or solvent is varied, but the 
amount of zinc chloride is kept constant. With zinc chloride the 
factor of safety is only about 2 ; with creosote it is in the neighborhood 
of 50. The reason for this great difference is that by diluting zinc 
chloride with water a depth of penetration as great as possible is 
obtained, and still only a small amount of zinc is used. On the other 
hand, it is impossible to give a deep oil penetration without using 
large quantities of oil. However, the usual oil penetration could be 
obtained, and at the same time creosote could be saved if it were 
diluted with some other oil, and this would have an effect analogous 
to that of the water in a zinc-chloride solution. As any good solvent 
of creosote would serve as well as another for this purpose, the main 
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