EVALUATION OF WOOD PRESERVATIVES 495 



molding between which he inserted a properly sized piece of untreated 

 wood, the whole being exposed over the surface of an agar culture of the 

 test fungus. He did not attempt to add a block of infected wood to his 

 setup as Trippo did; but his procedure illustrates an attempt of some 

 twenty years ago to test the protective action of preservatives in the 

 laboratory. His statement that sterilization might drive off a significant 

 amount of volatile preservative from freshly treated blocks, but that 

 the sterilizing process would probably have very little effect on the 

 preservative residual in weathered blocks anticipated similar views ex- 

 pressed in the present paper. 



For testing initial toxicities one can still use the Petri dish or agai 

 flask method; but it is about as unreahstic as Weiss's procedure as far 

 as tests of toxicity and permanence of wood preservatives are concerned. 

 The results can be presented for their academic interest, and the investi- 

 gator can keep safely aloof from the perilous practical problems of wood 

 preservation unless he attempts to translate his data into terms of 

 permanence and preservative value. Then his practical colleagues as well 

 as his technical friends point out to him, truly with a vengeance, the 

 error and unrealistic character of his efforts. 



It may be, as Rabanus^^ has suggested, that closely similar results can 

 be obtained by the agar and by the agar-block method in the case of 

 certain definite toxic chemicals, particularly water soluble ones. If so, 

 the Petri dish or agar-flask method could be used with such preservatives, 

 and the results of the tests could be applied in practice, after such agree- 

 ment between agar and block tests was firmly proven and established. 



It is completely unrealistic to attempt to arrive at significant values 

 for the volatile fractions of creosote by confining them within a tightly 

 closed culture dish. If such materials are really transient their toxic 

 function can operate only during the early life of the treated wood. In a 

 whole creosote, for example, the relatively higher toxicity of the volatile 

 low boiling fractions supplies an important initial power to the preserva- 

 tive, which power is evidently diminished as the volatiles leave the 

 treated wood. The degree of change in toxicity, as measured by the agar 

 method, in new and aged or weathered creosotes, as shown by the works 

 of Snell and Shipley,'^' Schmitz et al,'^'' '^' Baechler' and others is 

 distinctly reaHstic as an index of how an oil may be altered by time and 

 weather — at least with respect to a measure of its toxic properties. Such 

 changes have great practical significance when minimum quantities of a 

 preservative are employed, either for reasons of economy or in order to 

 insure cleanhness in the treated product. The trouble is that the results 

 of the Petri dish or agar-flask method do not indicate directly how much 



