BLEEDING OF WOOD. 267 



is held vertically. When the object is brought into a warm room, however, or, 

 still better, wrapped in a warm cloth, clear water gradually oozes out from the 

 wood at the lower cut end ; and if the latter is then held for some few minutes 

 through the opened window out in the cold air, the exuded water is plainly seen 

 to be slowly sucked in again, until the surface of the section appears distinctly 

 dry. If the cold piece of branch is placed in a cylinder of warm water at 

 25 — 30°C., so that only the upper section protrudes, water again oozes forth: this 

 occurs first at the most external layer of the wood and then progressively oozes 

 from the inner and older rings of the alburnum, in accordance with the progressive 

 warming from without inwards. Small bubbles of air come out of the vessels 

 with the water, and are forcibly ejected. If the object is lifted out and at once 

 brought in the same position into a cylinder of cold water, the layer of water on the 

 cut end then sinks again into the wood; again also progressively from without 

 inwards as the cooling effect penetrates, until the section appears dry. 



From this simple experiment several things are to be learnt. In the first 

 place, it is by the warming of the wood that water is expelled at the cut end; 

 and in fact the water is driven from the warm to the cold place. In accord- 

 ance with this, cooling brings about the re-absorbtion into the wood of the 

 expelled water. Secondly, detailed studies in the winter of 1859 showed me that 

 the quantity of water expelled always represents only a relatively small proportion 

 of the total quantity of water contained in the wood. It is obvious, and is 

 demonstrated by observation, that the phenomenon only appears at all when 

 the wood contains relatively large quantities of water. The poorer in water it is, 

 the more it must be warmed to bring about the emission of water. As Hofmeister 

 first recognised, basing his opinion on my observations, the whole phenomenon 

 depends chiefly upon the expansion of the air contained in the wood. We saw 

 in the lecture before last, that the wood-cells, even of wood very rich in water, 

 are never entirely filled with water; but that a portion of their cavity is occupied 

 by air-bubbles. It is the expansion of this air, saturated with aqueous vapour, 

 which forces out the water from the wood-cells ; and the cooling of these air- 

 bubbles, on the other hand, effects the suction by which the water is again drawn in. 

 That the change of volume of the water itself is far from sufficient to explain the 

 quantity driven out, I had already established by my investigation. Since, however, 

 the wood-cells are closed on all sides, and the vessels may here be neglected for 

 the time being, and since the phenomenon appears also in the wood of Conifers, 

 which has no vessels, the water driven out by the expansion of the air-bubbles 

 in the wood must obviously be forced through the cell-walls themselves; this, 

 of course, will take place most easily and rapidly through the thin portions of wall 

 which close the pits. With a slight rise of temperature, now, the force with 

 which the air-bubbles drive out the water is relatively feeble; and from the 

 fact that the water filters with facility through the wood cell-walls, we draw the con- 

 clusion that wood, even when it contains no vessels, must be in a very high degree 

 permeable. In how high a degree this is the case I convinced myself^ some 



» With respect to the extraordinary facility with which the filtration of pure water takes place 

 through fresh wood, further facts are also to be found in the treatise last cited. 



