A LABORATORY EVALUATION OF WOOD PRESERVATIVES 195 



tion of timber is still urgent. This is particularly vital in that creosote 

 is a loose term covering a congregation of compounds rarely twice the 

 same in quality or proportion. 



When in 1927, research on the development of a rapid means of 

 evaluating wood preservatives was initiated in the Chemical Depart- 

 ment of these Laboratories, primary consideration was accorded the 

 selection of the best available method for measuring the toxicity of 

 proposed preservatives against wood-destroying fungi. The technique 

 used was one which had been developed and extended to a considerable 

 degree by the workers at the Forest Products Laboratory in Madison, 

 Wisconsin. This petri dish method, described at some length by 

 Richards in 1923,^ was standardized in 1929 ^ at a conference of 

 American workers in St. Louis. Briefly, the method consists in adding 

 various amounts of the toxic agent under test to a nutrient medium in 

 the form of a hot malt-agar solution which is poured into a petri dish, 

 cooled, and the resulting gel inoculated with small sections of the hyphae 

 of a wood-destroying fungus (Fig. 1). The organism usually used is 

 culture no. 517 from the Forest Products Laboratory but others may 

 be chosen. The excellence of the preservative is based on the lowest 

 concentration which is able to kill or totally inhibit growth of the test 

 organism. 



While the petri dish method can be brought to a high degree 

 of efficiency, accuracy and precision by suitable precautions, it is 

 definitely Hmited in its practical application. It tells nothing of the 

 permanency of the material under test from the standpoint of leaching, 

 evaporation or chemical instability. Nothing is learned of the possible 

 reaction of the preservative with wood, and the dispersion in warm 

 liquid agar which later gels is a far cry from that obtained in wood. 

 There is an axiom of biological assay, that the substratum for in vitro 

 tests be as similar as possible to that encountered in nature. Neglect 

 of this principle in the field of antiseptics and germicides has been 

 responsible for many outstanding failures in vivo of materials which 

 had given brilliant promise in the culture tube. Doubt concerning 

 the validity of the petri dish test was substantiated when several 

 preservatives highly toxic according to this method failed in outdoor 

 exposure tests. In many cases such failures could not be ascribed to 

 obvious conditions such as high volatility or solubility. Instances 

 were also met wherein materials of little value according to the petri 

 dish method were able to prevent decay in the field. There is no dis- 

 position to advise against all use of this method as it is a valuable tool 

 in making initial judgments on a new material; but it should be verified 

 by other means before the expense of a field trial can be justified, and a 

 ^ Numbers refer to bibliography at end. 



