202 BELL SYSTEM TECHNICAL JOURNAL 



test in order to determine actual weight losses. Since oven-drying is 

 obviously a poor reference standard when volatile materials are under 

 consideration and may also bring about serious changes in the wood, 

 a constant humidity chamber is used for this purpose. An ordinary 

 bacteriological incubator kept at 30° C, fitted with slow-moving fans 

 and a shallow pan containing a saturated solution of common table 

 salt (Fig. 6), has proved to be completely satisfactory in this respect, 

 maintaining a relative humidity of 76 per cent with very little devia- 

 tion. The test pieces after treatment are placed on racks (Fig. 7) 

 and only a few days in the chamber are necessary for equilibration. 



Such a test method allows of three criteria as the basis for judging 

 the degree of attack. First, there is the amount and vigor of the 

 growth of the test-fungus on the wood block, readings of which are 

 made every four weeks. As this is difficult of expression, recourse is 

 had to the classical method of the serologists, wherein plus four denotes 

 the maximum. An attempt is made to evaluate both vigor and extent 



Fig. 7 — Test blocks and rack. 



of the fungal growth; the notation "2-4" would mean that the test 

 block was partly covered with a heavy mycelial mat, whereas "3-2" 

 would mean almost covered with relatively weak growth (Figs. 8 

 and 9). Often no growth occurs on the test piece and sometimes the 

 mycelial inoculum is actually killed. The second measure of extent of 

 decay is based on the loss of weight, with corrections for the effect 

 of leaching and evaporation during the test period, computed from 

 data obtained on treated controls put through the entire cycle without 

 inoculation. These controls are also of value in the empirical strength 

 rating made at the end of the test when the pieces are dissected in an 

 effort to judge the remaining strength — a rating of ten indicates no 

 detectable loss in strength as compared to the control and zero denotes 

 complete disintegration. 



Long experience with the petri dish method had emphasized the 

 high degree of specificity of the fungi to various toxics. No single 



