78 PROPERTIES OF WOOD. 



As regards so-called uninflammable wood, it is injected 

 with several chemical compounds, chiefly salts of alum, and 

 becomes difficult to kindle, but is still inflammable after being 

 subjected to lire for some time. 



10. Conductivity of ITood. 



(a) Heat. 



Wood is one of the worst conductors of heat and is there- 

 fore largely used for matches, and for handles of tools that 

 are subjected to various temperatures. Wood conducts heat 

 better longitudinally than transversely, in the ratio of 18 : 10 

 for softwoods and 13 : 10 for hardwoods. Heavy hardwoods 

 conduct heat better than softwoods, and wet wood better than 

 dry wood, as water is a better conductor of heat than wood. 



(#) Electricity. 



Wood is also a poor conductor of electricity and serves as 

 an insulator ; high specific weight and wetness reduce its 

 resistance to the passage of electric currents. This is the 

 reason why living trees are struck by lightning more fre- 

 quently than dry trees ; also isolated trees, on account of the 

 large amount of water they contain, their high specific weight, 

 and the spread of their crowns, than trees growing in dense crops. 

 Also certain species such as oak, in preference, as is generally 

 asserted, to beech. The latest investigations of Hartig, unfor- 

 tunately interrupted by his too early death, tend to show that the 

 immunity to lightning stroke assigned to beech is not war- 

 ranted. Hartig states that beech is just as often struck as 

 oak, but the external and internal action of lightning on 

 beech and oak differ. The fact stated by Janescu, that oily 

 trees (<'.</., beech, walnut, birch and lime) when compared 

 with starchy trees (oak, poplars, ash, elm) are worse con- 

 ductors and less frequently struck than the latter, should be 

 noted. [Kesinous conifers, such as Scots pine, are rich in 

 oil during winter, but poor in oil in summer. Oil is a bad 

 conductor of electricity. Tr.] 



(c) Sounil. 



Wood conducts sound well along its fibres, i.e., longi- 

 tudinally; the slightest noise at one end of a log can be 



