68 BULLETIN 1036, U. S. DEPAETMEISTT OF AGRICLXTURE. 



Humphrey and Fleming (£9), of the Forest Products Laboratory, 

 and Dean and Downes (30), of the Sheffield Scientific School, have 

 put this work on safer ground by using the timber-destroying fungi 

 in their tests. The former, who have done by far the greatest 

 amount of work and have used chiefly the fungi Fames annosus and 

 F. pinicola, place the toxic point of coal-tar creosote between 0.2 

 and 0.5 per cent. If this is the true toxic point, then one-fifth of 

 a pound of creosote per cubic foot of wood, provided it were prop- 

 erly distributed, would be sufficient to stop the growth of the fungi. 



CAUSE OF TOXICITY OF CREOSOTE. 



There is now little or no data on what compound or compounds in 

 coal-tar creosote are responsible for its toxic effect, because few of 

 the constituents have been isolated and subjected to tests. In 

 general, the constituents of coal-tar creosotes may be divided into 

 three groups — the hydrocarbon oils, the tar acids, and the tar bases. 

 Of the hydrocarbons that are found in coal-tar creosote, naphthalene 

 and anthracene, according to the methods used by J. M. Weiss (31), 

 seem to have very little toxic action against bacteria, yeast, and 

 penicilium. Russell and Pen<lleton (32) have shown that benzene 

 and toluene are capable of destroying certain undesirable (or harm- 

 ful) soil protozoa, bacteria, and fungi, but do not act on certain other 

 soil bacteria that are desirable. Naphthalene and crude oil, when 

 tested in the same way, gave a similar but slighter action. Weiss (31 ) , 

 Charitschkow (33), and Dean and Downes (30) have showTi that the 

 "neutral oils" — those oils that have been freed from tar acids and 

 tar bases by being washed with caustic soda and sulphuric acid — 

 still possess a considerable toxic property. Weiss (31) and Charitsch- 

 kow (33) state that the toxicity of the neutral oils is nearly as great 

 as the toxicity of the original creosotes from which they were obtained. 

 Dean and Downes (30) show that the acid-free oil is more toxic than 

 the original, but that the neutral oil (free from both acids and bases) 

 is only about two-thirds as toxic as the original. The same sort of 

 evidence has been obtained in the Forest Products Laboratory by 

 Huntley (34) , except that in his tests the toxicity of the neutral oils 

 exceeded the toxicity of the original creosote. There is, of course, 

 the possibility that the methods used to remove the tar acids and 

 tar bases were such that there were traces of these materials or their 

 salts still remaining in the oil, and that the increased toxicity of the 

 tar-acid free oil of Dean and Downes and of the neutral oil of Huntley 

 was due to the small amounts of tar-acid salts, which are somewhat 

 more soluble in the agar medium than are the acids themselves. 

 However, the weight of evidence now available seems to show that 

 the neutral oils of coal-tar creosote are themselves very toxic, and 

 that this toxicity is due to the lower-boiling hydrocarbons. 



