A LABORATORY EVALUATION OF WOOD PRESERVATIVES 207 



shorter time, but experience has shown that more consistent results are 

 obtained with the longer period. This is especially true in the case of 

 materials of moderate toxicity, wherein often little growth is seen for 

 two or three months, after which the fungus may become established 

 and rot the test piece. 



Using the routine technique, volatile compounds are often found to 

 be practically worthless. This is true of naphthalene, for instance, 

 but when special precautions are taken to insure its presence during 

 the exposure to the fungus, the effective toxicity of this hydrocarbon 

 cannot be questioned. Analysis of control blocks proved that the 

 naphthalene had evaporated quite completely from the test piece even 

 before sterilization. This difficulty can be surmounted satisfactorily 

 in the case of a single compound by injecting a generous quantity and 

 determining the actual amounts of material present from equilibrium 

 weights of the test pieces before treatment and just prior to inoculation. 

 Steam sterilization, of course, would introduce errors under such 

 circumstances, and while the risk of contamination with foreign 

 organisms is high, satisfactory results have been obtained with un- 

 sterilized blocks. In the case of volatile mixtures a similar procedure 

 permits the knowledge of the total evaporation before inoculation, but 

 determination of the loss of the individual constituents is practically 

 impossible. 



For all relatively volatile preservatives such as creosotes, the 

 regular method including sterilization can in a way be considered a 

 permanency test. Fortunately this evaporative loss is in the same 

 order of magnitude as that encountered in the field after an exposure 

 of several years, and correlation with outdoor tests is unexpectedly 

 good. Closer control of the amount evaporated would be desirable, 

 but experience has shown this to be difficult of consistent attainment. 

 Artificial weathering machines such as that described by Gillander, 

 Rhodes, King and Roche ^° constitute a reasonably successful attempt 

 to reproduce natural conditions. Leaching is more easily controlled 

 and duplicated than evaporation, and preservatives which by their 

 nature might be considered to be water soluble are subjected to a 

 standard leaching cycle if the initial test has shown them to be 

 promising. Again the close correlation of field results with the labora- 

 tory is gratifying. 



Of necessity, details of technique have been only sketchily reviewed 

 in this paper ; information as to the exact procedure will soon be avail- 

 able in the chemical press together with complete results on several 

 preservatives of interest. Table II contains the results of an assay 

 on a supposedly permanent inorganic preservative before and after 



