CHAPTER 11 



THE reoog:n'ition and classification of woods. 



Not only cariDenters and other workers in wood, but engineers, 

 surveyors and timber-merchants at present recognize the timbers 

 with which they are famihar as to kind, and even largely as to 

 quality, by methods obviously and confessedly empirical, mere 

 *' rule of thumb." From this it results that, though woods may 

 be accurately discriminated generically, as oak, ash, birch or 

 pine, the species are seldom correctly distinguished, and, as a 

 consequence, the best wood for any particular pur]30se is very 

 often not obtained. 



In these empirical identifications such more obvious but variable, 

 and therefore less trustworthy, characters as weight, hardness, 

 colour and odour are often more used than most of the struc- 

 tural characters described in the previous chapter. In attempting 

 a more thorough-going discrimination we cannot ignore these more 

 obvious characters ; but it is important to recognize their varia- 

 bility and consequently merely secondary importance. Details as 

 to the testing of weight and hardness will form the subject-matter 

 of a subsequent chapter : we are here only concerned with rough 

 approximations. 



Weight of wood. — ^The weight of wood depends mainly u|)on 

 two things, its compactness and its moisture. Compactness sig- 

 nifies the amount of woody or other sohd matter in a given bulk, 

 and this will generally be greater in slow-growing than in quick- 

 growing species, greater in heartwood than in sapwood. Moisture 

 is so far more variable in amount in the same wood, according to 

 the extent to which it has been naturally or artificially seasoned, 

 that no comparison of the weights of different woods can be of any 

 value unless the samples have been kiln-dried, and even by this 

 method it is difficult to secure a uniform ehmination of moisture. 

 If finely powdered and completely dried, all woods have a density 

 or specific gravity — a weight, that is, as compared to that of water 

 — of approximately 1*5. Many tables have been published giving 

 the density or specific gravity of various woods to three or even four 



34 



