112 BELL SYSTEM TECUMCAL JOURNAL 



of the norm with the percentage weight loss due to decay of the test block 

 may be used as a measure of the effectiveness of the preservative or toxic 

 material. An index of the value of a preservative treatment may be ob- 

 tained from the following computation: 



% loss of norm - % loss of treated block ^^^ 

 % loss of norm 



Values of the index would range from 100, representing complete protection 

 against decay, to 0, representing no protection whatever. 



In most cases, especially when no volatile preservative is present, the 

 untreated reference block disintegrates completely within twelve to six- 

 teen weeks. An inspection rating based on strength may be used to supple- 

 ment weight loss due to decay. The rating is made on the basis of appear- 

 ance and strength: 10 denotes a sound condition, 9 superficial decay, 8 

 superficial decay in spots or streaks, 7 general surface decay, 6 considerable 

 decay but not enough to allow specimen to be broken easily, 5 advanced 

 decay, and 4, 3, 2, and 1 different stages of advanced decay, determined pri- 

 marily by the ease with which the specimen is broken; denotes complete 

 disintegration. Ratings of 5 and below are considered failures. Similar 

 ratings have been used for sticks in field work. This method of rating was 

 used in the field trials of preservatives which appear later in this paper. 

 Although the system is an arbitrary one, considerable correlation has been 

 shown between these dissection ratings and the weight loss in percentage. 

 With a series of field sticks or blocks treated with the same low amount of a 

 preservative, ratings based on strength for the series are found to be very 

 closely correlated with weight losses, even when the ratings are made by dif- 

 ferent workers. 



The Influence of Moisture on Decay 



The first experimental factor studied was the moisture content of the wood 

 preceding and during the time that decay took place. Statements in the 

 literature concerning the optimum moisture content for the decay of wood 

 have placed the figure variously from fiber saturation, 27-30% to 60% 

 (Schorger, 1926)1" and 150% (Benton & Ehrlich, 1940)ii of the wood sub- 

 stance based on the oven-dry weight. 



Figure 1 gives data on the moisture content of blocks exposed to the seven 

 species of fungi for periods of one, two, three, and four months. Un- 

 inoculated control blocks, removed at the end of each of these periods, were 

 found to be at filer saturation, indicating 100% relative humidity in the 

 bottles and Uttle or no migration of Uquid water. 



During the progress of decay there is a rapid decrease in the weight of 

 the wood substance. But the amount of water present in each block does 



