TIMBER PHYSICS 143 



timber. Laboratory investigation into the varia- 

 tions of structure and mechanical properties in 

 timbers from the same kind of tree can show us 

 how far these variations are due to the different 

 silvicultural conditions under which the timbers 

 were produced. A considerable amount of work 

 has already been done, but in such investigations 

 complete results are exceedingly difficult to get. 



Much of the timber used (over 40,000,000 in 

 value a year) is used for outdoor work such as 

 fencing posts, railway sleepers, telegraph poles, 

 mining timber, etc., where it is liable to decay. 

 The high price of the timber and the steadily 

 rising demand make the question of increasing the 

 durability of the timber one of vital importance. 

 It is clear that if the life of the timber could be 

 doubled, only half as much timber would be needed. 

 Green timber decays most rapidly and hence the 

 first preservative measure was to season it before 

 use, but no matter how well seasoned the wood 

 may be, if it be exposed to outside conditions it 

 can again absorb moisture and so be exposed to 

 disease as before. For some time now it has been 

 customary to impregnate the timber with sub- 

 stances which render the cell contents unsuitable 

 for fungus growth, and which coat the cell walls 

 and prevent their absorbing water. The processes 

 employed are various and the substances which 

 have been most extensively employed are creosote, 

 zinc chloride, bichloride of mercury, and copper 



