HARDNESS. 35 



will be the resistance it affords to instruments ; its hardness 

 is therefore proportional to its specific gravity, and heavy woods 

 are harder than light woods. 



' 2. Coherence. 



A firm coherence of the woody fibres as opposed to loose 

 texture will evidently increase the hardness of a wood. Medullary 

 rays are of considerable influence here, but observation has not 

 yet decided, whether the coherence of the ligneous linings of cell 

 walls with the primary wall common to two contiguous cells is 

 of importance in this respect, and whether it varies for difi"ereut 

 kinds of wood. The course of the fibres being straight, curved 

 or twisting, has certainly, however, some influence on their 

 coherence. 



3. Resin* 

 Coniferous woods become harder the more resin they 

 contain, especially when this fact is combined with narrow 

 annual zones of wood. Eesin also increases the hardness of 

 a wood the more, the less turpentine it contains, i.e. the 

 harder it is in itself [thus Weymouth-pine containing much 

 turpentine but little rosin is, especially when dried, a very soft 

 wood. — Tk.] All this explains the great hardness of knots in 

 larch and spruce planks, for these knots are very narrow-ringed, 

 and highly resinous. 



[Oil applied to cricket-bats increases their toughness and resistance 

 to blows, and should be re-applied when dried. — Tr.] 



4. Def/rees of Moisture. 

 Dry wood is harder than green wood, a fact which may be 

 partly explained by the softening of the fibres by water, 

 and partly by the swelling of wet wood. Heavy woods gain 

 most in this respect, and it is well known that beech, oak and 

 sycamore wood is much easier to carve, hew and saw when green 

 than when dry. Pliability also results from moistening wood ; 

 wood-fibres in this condition bend before the intrusive edge of 



* [Resin is a mixture of oil of turpentine and rosiu. — Tr.] 



D 2 



