380 FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



Conceived in this manner it becomes tho pivotal science of the art of forestry, around which the practice botli 

 of (lie consumer and producer of forest growth moves. 



The first part of our science would require a study into gross and minute anatomy, tho structure of the wood, 

 form, dimensions, distribution, and arrangement of its cell elements and of groups of structural parts, not only in 

 order to distinguish the different woods, but also to furnish the basis for :in explanation of their physical and 

 mechanical properties. We next would class here all investigations into tho physical nature or properties of tho 

 wood material, which necessarily also involves an investigation into the change, of these properties under varying 

 conditions and influences. A third chapter would occupy itself with the chemical composition and properties of 

 woods and their changes in tho natural process of life, which predicate the fuel value and durability as well as the 

 use of the wood in chemical technology. 



Although, philosophically speaking, it would hardly seem admissible to distinguish between physical and 

 mechanical properties or to speak of "mechanical'' forces, for the sake of convenience and practical purposes it is 

 desirable to make the distinction and to classify all phenomena and changes of nonliving bodies, or bodies without 

 reference to life functions, into chemical, physical, and mechanical phenomena and changes. As chemical phenomena 

 or changes, and therefore also conditions or properties, we class, then, those which have reference to atomic struc- 

 ture; as physical phenomena, changes, and properties those which refer to and depend on molecular arrangement, 

 and as mechanical (molar) changes and properties those which concern the masses of bodies, as exhibited under the 

 influence of external forces, without altering their physical or chemical constitution. 



There is no doubt that this division is some what forced, since not only most or all mechanical (as here conceived) 

 changes are accompanied or preceded by certain alterations of the interior molecular arrangement of the mass, but 

 also many physical phenomena or properties, like density, weight, shrinkage, having reference to the mass, might 

 lie classed as mechanical; yet if we conceive that physical phenomena are always concerned with the "quantity of 

 matter in molecular arrangement" and with the changes produced by interior forces, while the latter are concerned 

 rather with the "position of matter in molecular arrangement " and with changes under application of exterior 

 forces, the distinction assumes a practical value. 



Our conception of these distinctions will be aided if we refer to the physical laboratory as furnishing the 

 evidence of physical phenomena, and to the mechanical laboratory as furnishing evidence of mechanical phenomena. 



These latter, then, form the subject of our second or dynamic part of timber physics, which concerns itself to 

 ascertain mainly by experiment, called tests, under application of the laws of elasticity, the strength of the material 

 and other properties which are exhibited as reactions to the influence of applied stresses, and those which need 

 consideration in the mechanical use of the material in the various arts. 



Having investigated the material in its normal condition, we would necessarily come to a consideration of 

 such physical and chemical conditions of the material as are abnormal and known as disease, decay, or defects. 



Finally, having determined the properties and their changes as exhibited in material produced under changing 

 conditions or differing in physical and structural respects, it would remain the crowning success and goal of this 

 science to relate mechanical and physical properties with anatomical and physiological development of the wood 

 substance. 



The subject-matter comprised in this branch of applied natural science, then, may be brought into the following 

 schematic view: 



TIMBER PHYSICS, OR THE SCIENCE OF WOOD. 



I. WOOD STRUCTURE OR XYLOTOMY. 



(a) Exterior form. 



Here would be described the form development of timber in the standing tree, differentiated into root 

 system, root collar, bole or trunk crown, branches, twigs; relative amounts of material furnished by each. 

 (l>) Interior structural appearance; differentiation and arrangement of groups of structural elements. 



Here would be described the gross structural features of the wood, the distribution and size of medul- 

 lary rays, vessels, fibro-vascular bundles, as exhibited to the naked eye or under the magnifying glass on 

 tangential, radial, and transverse sections; the appearance of the annual rings, their size, regularity, dif- 

 ferentiation into summer and spring wood, and all distinguishing features due to the arrangement and 

 proportion of the tissues composing the wood. 

 (o) Minute anatomy or histology; differentiation and arrangement of structural elements. 



Hero the revelations of the microscope are recorded, especially the form, dimensions, and structure of 

 the different kinds of cells, their arrangement, proportion, and relative importance in the resulting tissues. 



(d) Comparative classification of woods, according to structural features. 



(e) Laws of wood yrowtli with reference to structural results. 



Discussion of the factors that influence the formation of wood in the standing tree. 

 (/) Abnormal formations. 



Burls, bird's eye, curly, wavy, and other structural abnormities and their causes. 

 II. PHYSICAL PUOPEKTIES, i. e., properties based on molecular (physical) constitution. 



(a) Exterior appearance. 



Such properties as can be observed through the unaided senses, as color, gloss, grain, texture, smell, 

 resonance. 



(b) Material condition. 



Such properties or changes as are determined by measurements, as density or weight, water contents 

 and their distribution, volume, and its changes by shrinkage and swelling. 



