TIMBER TESTING 295 



tJaan in tlie use of a saw, therefore less moisture 

 is driven ofi. 



ThorougKly over -dry wood retains a small 

 percentage of moisture in the cell walls, but all 

 the free water which may be found within the 

 cells is driven off in the drying process. " Dry- 

 ing produces a decided increase in the strength 

 of wood, particularly in small specimens. An. 

 extreme example is the case of a completely dry 

 Spruce block, two inches in section, which will 

 sustain a permanent load. four times as great as 

 that which a ' green ' block of the same size 

 will support." 



Obviously it is important in carrying out 

 any timber testing that due attention should be 

 paid to density. In comparing equal bulks of 

 various dry woods, the heavier woods contain 

 more wood substance than the lighter ones. 

 Various resins and gums which occur in the 

 wood tissues may, however, make the sample 

 with the smaller amount of wood substance 

 appear the heavier. Although practically all 

 woods are considerably heavier than water, it 

 may appear anomalous that the wood should 

 float on water. . An examination of any piece of 

 wood cut across the grain wiU show that it is com- 

 posed of numerous minute tubes, whose struc- 

 ture and function have already been described. 

 These tubes are, for the most part, filled with 



