CHAPTER m. TOXIC PROPERTIES OF COAL-TAR CREOSOTES. 



To. be an efficient wood preservative, any oil must have at least 

 two properties: It must be toxic; that is, able to destroy or to inhibit 

 the growth of the organisms that cause the destruction of timber, 

 and it must be as nearly permanent as possible. In addition to these 

 two, it may have other properties, such, for example, as that of 

 waterproofing, which would aid in the retardation of fungous growth. 

 The most efficient wood-preserving oils, therefore, must possess a 

 high degree of toxicity combined with a high degree of permanence. 

 The long service of timbers treated with coal-tar creosote shows 

 conclusively that coal-tar creosote possesses these properties to a 

 marked degree. 



Toxicity tests may be divided into three groups or classes: (1) tests 

 on timber-destroying fungi; (2) tests on marine borers; and (3) tests 

 on insect borers. The first two are of the greatest importance to the 

 wood-preserving industry at the present time. The tests on fungi 

 may be conducted in at least three ways: (1) by Petri-dish tests, 

 which require only a short time for their completion; (2) by fungus- 

 pit tests, requiring a somewhat longer time; and (3) by service tests, 

 requiring from 4 to 10 j^ears to obtain the results. The last is, of 

 course, the most conclusive proof of the preserving value of any 

 wood-preserving oil, but requires an exceedingly long time, and is, 

 moreover, a test that combines permanence and toxicity. Further- 

 more, in such tests not infrequently the wood fails mechanically 

 before the usefulness of the preservative is ended. The first and 

 second are only comparative tests and show what might be expected 

 of any wood preservative when compared with some other material 

 of known value. In these two tests the factor of permanence is 

 largely but not completely eliminated; it is, therefore, possible that 

 a material which under tests promises to be a very effective pre- 

 servative may not prove to give good service because of its lack of 

 permanence. 



FUNGUS-PIT TESTS. 



Fungus-pit tests have been started by the Forest Products Lab- 

 oratory, by Chapman {28), of Westinghouse, Church, Kerr & Co., 

 and by Hosford, of the American Telephone & Telegraph Co., and 

 probably by others. At the Forest Products Laboratory the tests 

 consisted of placing blocks treated with various preservatives in 

 a concrete pit in which the himiidity and temperature could be 



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