Preservative Treatment of Timber. 7 



thick-walled cells of the so-called "summer" or "autumn-wood" 

 (Sr.), which are formed during the last part of the year's growth. 

 (See Figs, i and 4.) The larger thin-walled cells of the so-called 

 "spring-wood" (Sg) were unruptured and therefore, according to 

 Tiemann's theory impervious to gases and heavy oils. Yet as 

 Tiemann has figured clearly,* air passes very rapidly through 

 the thin-walled cells of dry "spring- wood."' 



C. Distribution of "Slits" in the Layers of the Cell Wall. 



The wall of a wood cell is not chemically and stucturally ho- 

 mogeneous throughout, but is differentiated in most cases into 

 three more or less clearly defined layers or coats. The outer 

 membrane or layer which is supposed to entirely inclose the rest 

 of the cell is called the primary wall, and is considerably thinner 

 than the succeeding layer within, which is heavily lignified and 

 very variable in thickness (Fig. 4). The latter layer is called 

 the secondary wall and is separated from the lumen or cavity of 

 the cell by a third, usually extremely thin, layer or tertiary wall 

 (Fig. 7). The secondary wall is comparatively thin in the large 

 "spring-wood" cells, but becomes much thicker in the cells formed 

 at the end of the year's growth. (See Fig. 4). As has been 

 stated, cracks or "slits" due to drying are confined to the heavy 

 walled cells of the "summer-wood" (See Sr. Figs, i and 4). If 

 very thin, carefully cut and stained sections are examined under 

 high powers of magnification it becomes evident at once that the 

 so-called "slits" or rents in the cell walls are confined entirely 

 to the secondary and tertiary layers (See Fig. 4). The primary 

 walls remain unruptured preventing effectively the penetration of 

 gases and heavy oils from one cell to another by means of the] 

 cracks in the secondary ivalls. In Fig. 4, a cross section of 

 pine wood, it may be noted that in order for air to pass from one 

 cell cavity to another it must pass through two layers of unrup- 

 tured primary wall, which together possess considerable thick- 

 ness, equal in many places to the thickness of a single layer of 

 secondary wall. A similar condition is illustrated in Fig. 2, a 

 cross section of the fibers of a "hardwood" or broad-leaf tree. 

 From this it is clear that drying cracks or "slits" even when pre- 



*Tiemann, H. D. The physical structure of wood in relation to its 

 penetrability by preservative fluids. Fig. 5, page 8. 



