599 



[Rothrock. 



Some Microscopic Distinctions hetioeen Good and Bad Timber of the Same 

 Species. By Dr. J. T. Rothrock. 



{Read before the American Philosophical Society, February 2, 1S83.) 



A cross section of one of our ordinary "hard woods " shows, more or less 

 conspicuously, pores which are known as ducts, and which from tlieir rela- 

 tivel}^ large size are distinctly visible to the naked eye ; secondly, it 

 shows much smaller pores which may, or may not, require the magnifying 

 glass to detect, and whose walls constitute the woody fibre of the stick ; 

 thirdly, we should have (assuming the specimen to be an exogen), the an- 

 nual rings which mark, as a rule, the limit of each year's growth ; fourthly, 

 there would be the radir^ lines extending from the centre outwardly to the 

 bark, these being the medullary rays or the so-called "silver grain." 



If, on the other hand, the specimen under observation were one of our or- 

 dinary cone bearing trees, the ducts would be wanting, and the mass of the 

 section would be composed of woody fibie. There may be openings wh'ch 

 will resemble the ducts in hard wood, but instead of showing regularly or- 

 ganized walls, these will be found to repvesent simply openings left by the 

 destruction or the separation of the woody fibres. They are by no means 

 so numerous ordinarily as the ducts in an average "hard wood stick." 



Considered from the standpoint of resistance to longitudinal strain, the 

 strength-giving element of wood is the woody fibre ; and other things being 

 equal, it is strong in proportion as the fibre walls aie relatively thick, and 

 the fibre cavities relatively small. lUusvrating this, we have the following 

 cross sections of wood fibres, all magnified 242 diameters : 1, is that of Abies 

 subalpina (Pumpkin Pine) from Utah ; a timber which is almost worth- 

 less ; 2, is that of our American Linden ; 3, represents the Butternut (or 

 Juglans cinerea) ; 4, is the Pig-nut Hickory (or Carya porcina) and 5, is 

 that of an average specimen of White Oak fibre (Quercus alba). Consider- 

 ing the areas of the cavities in each of these sections, the White Oak has 

 about six times as much wood in its walls, as there is in that of the Pump- 

 kin Pine — a fact which it must be allowed will go far toward explaining the 

 difierences in the strength of the two woods. It is true that there may be 

 difierences in the strength of wood which are due to the molecular difl"er- 

 ences involved in the structure of the fibre, but with these we are probably 

 in no position to deal. The intercellular substance which is destroyed by 

 boiling in nitric acid and potassium chlorate is to a certain extent an ele- 

 ment in the strength of wood. There can be no doubt but that it aids in in- 

 creasing the friction between the individual fibres, and is therefore the chief 

 agent by which these are bound together, and thus resist longitudinal strain. 

 So far as my investigations go, there is less relation between length of fibre 

 and strength, than there is between thickness of fibre wall and strength. 

 Some woods acquire additional strength, both longitudinal and transverse, 

 from a twisting of the wood fibres among themselves. The Rock Elm is a 

 notable example of this among our larger trees ; as the Viburnum nudum 

 or Withe-rod is among the shrubs. 



