68 BULLETIN 1036, U. S. DEPARTMENT OE AGRICULTURE. 
Humphrey and Fleming {29), 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 Fomes 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 Pendleton (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 shown 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. 
