Unprofessional Forestry. 229 



however, when the water contents sinks below normal air-dryness 

 (15%), hardness increases only slowly, and sometimes decreases 

 even, owing to a loss of shearing strength or increase of cleavability. 

 (? Ed.) An interesting reference is here made to the recognition 

 of this fact by Duhamel de Monceau, who in his Traite de la con- 

 servation et de la force, published in 1780, says: "Dry wood is to be 

 employed; but it must still contain some moisture, in order to be 

 hard, from which I conclude that too dry Avood is not able to do 

 good service." Hence hardness tests, like all other tests, should be 

 made with normal moisture contents (15%). [Or, much better on 

 green or watersoaked material, when this disturbing factor is elim- 

 inated , the relabivity ceasing beyond 32%. — Ed.] 



Incidentally the difference in hardness of excentrically-grown 

 conifers was tested. The excentricity, due to one-sided wind pres- 

 sure, consists not only in increased ring width, but in increased 

 production of fibres (red wood) ; the wide-ringed side is the harder, 

 yet, according to Schwappach, in this wood formation, although 

 heavier, the compression strength is smaller. 



The influence of impregnation with heavy oils (creosote) was 

 made a special study, using pavement blocks, some treated, some un- 

 treated, some kept in a room, some for two and a half years exposed 

 to the weather. A slight decrease in hardness was generally ob- 

 served, with the exception of spruce and larch, two species difficult 

 to treat and hence not thoroughly impregnated, which showed slight 

 increase. This decreasing of hardness due to the oil is, however, off- 

 set by an increase due to drying, for Avhile shortly after the im- 

 pregnation the treated wood showed smaller strength than the un- 

 treated, after 2 1/2 years of drying the reverse relation was observed. 

 The untreated wood, left in the weather lost hardness, however, even 

 though no fungus was as yet visible, while the treated wood retained 

 its hardness. Untreated beech wood, especially, exposed for 2 1/2 

 years, lost more than 70 per cent of hardness and strength, being 

 thoroughly rotten; while the loss in treated wood was only 20% of 

 that of the loss in hardness and 30 % of that of tRe loss in the com- 

 pression strength of untreated wood. 



The range of hardness within the species tested was 1.4 for palm 

 and 15.6 for ebony, these figures expressing kilogram per 1 square 

 milligram. 



It is also proposed to base a scale of hardness on the quotient 



— ^^fi*^"^^? (Kg.) ■ j^^^ wood being the most resistent and most desir- 

 speciflc weight (xlOO) ° 



able which contains the greatest hardness with lowest weight. 



