80 BULLETIN 1036, U. S. DEPAKTMEIS'T OF AGRICULTUEE. 



thing is to obtain the cheapest material that will meet the conditions. 

 Benzene and chloroform would answer the requirements so far as 

 physical and chemical conditions are concerned, and on drying they 

 would evaporate just as water does; but their use is, of course, out 

 of the question on .account of their great cost. Wood-preservers 

 are practically confined by cost to such crude raw oils as crude 

 petroleum, coal-tar, or water-gas tar. Of these three, coal-tar and 

 water-gas tar are better solvents of creosote than is crude petroleum. 

 Of the two tars, water-gas tar seems for the following reasons to fulfill 

 more nearly all the requirements of a diluent: 



(1) It contains an exceedingly small amoimt of suspended matter, 

 whereas coal tar, as a minimum, has about 3 or 4 per cent. The 

 viscosity of water-gas tar is usually less than that of coal tar. A 

 mixture of water-gas tar and creosote may, therefore, be expected to 

 penetrate more rapidly than an equal mixture of coal tar and coal-tar 

 creosote. 



(2) It may be used in its raw state when it is free from water. 

 Coal tar containing ammonia was found by Bolton to be injurious to 

 wood; therefore it must be refined in order to remove the ammonia 



(3) Crude water-gas tar, water free, is cheaper than refined coal 

 tar. 



Inasmuch as service records (27) show that the failures so far 

 experienced in ties, poles, and other timbers have been the result 

 of mechanical wear, checking, and similar causes, and not of the 

 failure of the preservative, it is apparent that the limit of the life of 

 coal-tar creosote has not been reached in this kind of service. In 

 other words, the factor of safety is probably very much larger than it 

 need be. It would seem justifiable in such conditions to dilute the 

 creosote with some cheaper material, such as tar, to make the life 

 of the preservative and the mechanical life of the wood more nearly 

 equal. The best kind of oil to use for dilution under such conditions 

 would be the low-boiling creosotes, because these contain the greatest 

 amount of toxic materials. If the theory of the mechanism of pre- 

 servative action suggested in this bulletin is correct, these low- 

 boiling creosotes probably need more high-boiling materials to retard 

 their too rapid solution. 



PROPERTIES OF TAR-CREOSOTE SOLUTIONS. 



But little can be said about the properties of tar-creosote solutions 

 except that they should be intermediate between the same properties 

 of creosote and tar. A small amount of work has, however, been 

 done and the results of it are given below: 



SPECinC GRAA'ITY. 



The addition of a heavier substance, such as tar to creosote will, 

 of course, increase the average specific gravity of the solution. This 



