A NATIONAL PLAN FOR AMERICAN FORESTRY 1389 



operations. What little has been accomplished in theory and tech- 

 nique has not been adequately applied to wood as a construction 

 material where control of acoustic properties is required. Further 

 knowledge of the electrical properties of wood might seem to be unim- 

 portant in direct application to uses of wood, since wood in its un- 

 modified state is not reckoned as either a good conductor or a true 

 insulator. Research on electrical properties has, however, been of 

 great assistance in solving other problems, such as the measurement 

 of moisture and the determination of the submicroscopic pore volume 

 of wood, so that, indirectly at least, further knowledge of electrical 

 properties would be of value. 



The great importance of a complete and detailed scientific know- 

 ledge of hygroscopicity is evident from the fact that it affects every 

 other physical and mechanical property of wood. The water in wood 

 occupies two different kinds of cavities, the microscopic cavities and 

 the extremely small spaces between the submicroscopic structural 

 units. The larger cavities remain as cavities of the same size whether 

 they contain water or not, but the smaller cavities decrease in size 

 according to the amount of water removed from them. The lowering 

 of the vapor pressure of the water in the latter is the basis for the 

 hygroscopic property of wood its ability to absorb water from the 

 air and their change in size is the basis for the swelling and shrinking 

 of wood with change in content of hygroscopic water. 



This hypothesis of the cause of hygroscopicity and shrinkage is 

 tentative and imperfect because there are so few facts from which to 

 develop it. For a satisfactory understanding of these phenomena, 

 intensive research is required on such diverse subjects as the hygro- 

 scopicity of different components of wood, directional shrinkage of 

 the structural units, hysteresis effects, absorption of other liquids 

 than water, and diffusion of hygroscopic water. 



GROWTH CONDITIONS 



The proper and satisfactory use of wood has had to depend on selec- 

 tion from a widely varying natural product in order to obtain the 

 different kinds of material required. In this respect it is fortunate 

 that wood is a widely varying product. From another point of view, 

 the use of wood would be less a problem if its properties and minute 

 structure ^could be controlled in somewhat the same manner as the 

 metallurgists controls the quality of metals in manufacture. 



Such control of the raw material, wood, lies in controlling the con- 

 ditions of its growth. Distinctive species characteristics cannot be 

 changed, but within the ordinary variations of a species wood of 

 more uniform and more desirable properties can be grown. 



The existing information on this subject is very slight in view of 

 the wide field to be covered. There are so many species, properties, 

 and growth conditions that the complete correlation of all or even the 

 most important of them will take a large amount of research. It has 

 been found that with longleaf pine the proportion of summerwood to 

 springwood (and hence density and strength) can be varied within 

 limits according to the amount of soil moisture available. In second- 

 growth southern pines the increased rate of growth that accompanies 

 increased openness of stand causes a decrease in the density and 

 strength of the wood, and the indications are that this relationship 

 holds for the softwoods as a class. In hardwoods, on the other hand, 



