134 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



the temperature and humidity conditions of the incubation room, the 

 growth of the fungus mycelium covers the feeder blocks in about two 

 weeks and the fungus threads are then well started downward into the 

 soil. 



Treated test blocks are weathered and then conditioned under con- 

 trolled temperature and humidity to approximate constant weight. They 

 are then sterilized, along with untreated control blocks, in an autoclave 

 for 15 minutes at 100°C, atmospheric pressure. 



As a rule two treated blocks having approximately the same retention 

 of preservative are placed together in a single test bottle. The incubation 

 period is three months, in an incubation room held at a temperature of 

 80 zb 2°F and at a relative humidity of 70 dz 2 per cent. At the end of 

 this period the cultures are taken down. This means that the blocks are 

 removed from the bottles, brushed free of fungus mycelium, and weighed 

 immediately. They are given a preliminary examination for decay evi- 

 dence, and then reconditioned, under the same temperature and humid- 

 ity conditions as before sterilization, to approximate constant weight. 

 Fungus attack is determined by observation and by weight losses. 

 The general setup of the cultures is illustrated in Fig. 1 (a-d). 



Inoculation and Incubation Rooms 



To faciUtate handUng the soil-block cultures, an inoculation room and 

 an incubation room have been built (Fig. 2) at the Murray Hill Labora- 

 tories. Both are held at approximately the same temperature and rela- 

 tive humidity, that is, 80°F and 70 per cent. The inoculation room 

 senses as a lock chamber, and passage from it to the incubation room 

 has a negligible effect on the humidity and temperature of the incuba- 

 tion room. The latter is provided with an illuminated double plate glass 

 window (Fig. 3), so that the interior can be exhibited without the neces- 

 sity of entering the room. This window is fitted with a heavy roller 

 shade, and the room ordinarily is kept dark. 



Soil Characteristics and Moisture Content 



The question that is asked most often about the cultures is whether 

 a standard soil is used. European and American criticism has been defi- 

 nitely directed^^' ^^ at the fact that the use of different soils might have 

 so much effect upon the growth and the reaction of the test fungi in the 

 cultures that quite different results would be obtained by investigators 

 in different laboratories. This possibihty is recognized; but the evidence 

 to date seems to point to the general conclusion that perhaps the prin- 



