1388 A NATIONAL PLAN FOR AMERICAN FORESTRY 



and dyestuffs being common amongst them. While the detailed 

 composition of the extractives in some few species is well known, it 

 is incomplete or entirely lacking in most. 



MECHANICAL AND PHYSICAL PROPERTIES 



The mechanical characteristics of wood its strength, elasticity, 

 and related properties depend on the physical properties of the 

 structural units, their arrangement, and mode of joining. The 

 structure of wood is so complex, however, and the structural units 

 so small, that the mechanical properties have not been actually 

 determined in any such manner, but instead have been directly 

 measured by standard engineering testing methods. The various 

 important strength values of the 160 principal American woods have 

 been determined and general rules developed for the effects of density 

 and moisture content. This kind of information is fairly complete, 

 although more information on the range of values as well as the 

 average values, and on second growth as well as virgin timber, would 

 be desirable. There are also two important commercial properties, 

 resistance to abrasion and workability under tools, for which no 

 figures are available. 



The information that we possess as to strength properties has been 

 collected with only incidental reference to structure; the direction 

 (longitudinal, radial, or tangential) in which the force was applied 

 was commonly known, and one structural characteristic, density, was 

 always determined. The finer details of structure were not deter- 

 mined, however, nor were the tests designed to show the effect of 

 structural variations in any minute degree. This kind of work, only 

 recently undertaken, has naturally begun with the influence of the 

 largest unit structures, the two layers of the annual ring, spring wood 

 and summer wood, upon the strength of the piece. Much remains 

 to be done even in this field of gross structure, and then more com- 

 plicated fields of smaller units, such as the thickness of the cementing 

 layer of lignin and the slope of the spiral angle of the fibrils, must be 

 developed before scientific knowledge of the relation between structure 

 and strength can be considered at all adequate. 



There are other types of scientific details in wood mechanics, 

 knowledge of which would be very desirable. For instance, wood is 

 not truly elastic but has a tendency toward gradual plastic yielding, 

 and it is important to know whether the plasticity has its origin in 

 the cellulosic fibers, in the lignin cement, or in shear between the 

 fibers. 



Other physical properties as distinguished from the purely mechan- 

 ical are also obviously dependent upon the minute structure of wood. 

 Among these, heat, acoustics, electrical properties, and hygroscopicity 

 are outstanding. None has received thorough or systematic study. 



The handbook figures for heat conductivity of wood are incomplete 

 as to species, moisture content, direction of the grain, density, and 

 temperature boundaries. A few accurate determinations have been 

 made on the effect of some of these factors, but not enough to give 

 the architect or engineer the specific information he requires in order 

 to determine what wood to use or whether to use wood. Heat con- 

 ductivity also has important bearings on the fire resistance of wood 

 in large sizes, on wood distillation processes, on the preheating preced- 

 ing impregnation treatments, and on other important industrial 



