expansion and contraction of timber , 273 
It appears probable that, by means of this kind of expan- 
sion, the internal parts of timber trees so frequently become 
rifted or cleft. Winds have been assumed by some, and 
frost by others, as the cause of these injuries. But winds 
cannot possibly be the cause, as pollared oak trees, upon 
which these can exert but very little power, are almost al- 
ways rifted ; and the frost of this climate is rarely, or never, 
sufficiently intense to congeal the winter sap of trees. This 
agent must also, I conceive, act suddenly, if it act at all, and 
the trunks of large oaks can not suddenly be cleft asunder 
in silence. The oak timber of England is also much more 
frequently rifted than that of the north of Europe. The force 
with which the cellular substance of timber expands, is fully 
equal to produce the preceding effects. I have often seen it 
overcome the pressure of many tons : it is therefore greatly 
more than equal to give the impulse to the sap, which was ob- 
served by Hale ; and as it is obviously in action in the living 
tree, 1 must retain the opinion which I formerly gave, that it 
is the agent by which motion is given to the ascending fluid. 
How it immediately acts upon the passages through which that 
fluid ascends, and whether that fluid passes through the cells 
themselves, or through the intercellular passages described in 
the elaborate w r ork of Dr. Kieser,* I confess myself to be 
wholly ignorant, and the slow motion of the fluid, the exces- 
sive minuteness of the passages, and the varieties of directions 
in which it is often moving at one and the same time, will 
ever render this a question of extremely difficult solution. 
There is another kind of contraction in timber whilst drying, 
and of expansion when subsequently wetted or moistened, 
* Memoire sur POrganization des Plantes. 
N 11 2 
