HARDNESS. 49 



easily worked wet than when dry. In soft woods, especially 

 those of broadleaved trees, the wetting ncreases the tenacity 

 more than the looseness of texture, so that they are easier to 

 work dry than wet. 



4. Specific weight. This is a measure of the mass of 

 wood in a given volume, so that wood of high specific weight 

 is hard. The future discussion of specific weight therefore 

 applies also to the hardness of wood. 



5. Parts of a tree. Hardness here corresponds with 

 specific weight. The softest wood is root-wood; then the 

 western and eastern sides of the bole ; stump-wood ; the 

 upper side of the branches ; the lower side of the branches, 

 which last is the hardest wood that a tree produces. Spring- 

 wood is always softer than summer-wood, especially when 

 the latter is particularly broad. A more detailed account of 

 this question will be given hereafter (p. 54). 



6. Coherence, owing to the union of the cells and of the 

 materials that form their walls. Differences in coherence, in 

 spite of a similar specific weight in the woods, cause a consider- 

 able difference in the resistance offered by them to implements. 



7. Presence of substances other than water. If water, 

 which softens it, has left the cell-wall, and another material, 

 such as resin or any impregnating substance, has replaced it, 

 the wood becomes harder. Thus highly resinous wood, such 

 as the knots of conifers, is extremely hard. 



8. Temperature. Frozen wood is much harder than 

 unfrozen wood. The slipping aside of wedge and axe in work- 

 ing frozen wood cannot be explained in accordance with the 

 prevalent theory, if, when wood is frozen, water is driven from 

 the cell-wall. 



Owing to the connection between hardness and specific 

 weight, more detailed data will be given under the latter head- 

 ing. Here the following scale of hardness is suggested : 



Very hard, hard as a bone. Pulley-wood, ebony, iron wood. 



Hard. Box, pitch-pine, hickory, barberry, hornbeam, oak, 

 robinia, field-maple, mahogany, ash, beech, sweet-chestnut. 



Fairly hard. Walnut, pear- and apple-wood, elm, larch, 

 yew, cherry-wood, birch. 



Rather soft. Alder, horse-chestnut. 



F.U. E 



