*39 
of the heart wood of trees. 
quantity of organizable matter to generate a new layer of 
bark over the whole surface of its alburnum, and it thus 
adapts its organization to its more permanent state of 
existence. 
The alburnum and bark of trees of some species did not 
appear to me to be capable of containing in winter so much 
organizable matter as I saw thus expended in particular 
cases ; and I found the thickness of both these substances to 
be subject to great variation in trees of the same species, and 
of equally vigorous growth. I also observed that the heart 
wood of the oak, and of other trees, always contained much 
uncombined soluble matter ; and that its external layers 
annually acquired greater specific weight and density, owing 
to the reception of new matter, which must, I conceived, have 
been conveyed into it in a fluid state. I was therefore led to 
ascertain what quantity of fluid is retained by the heart wood 
of the oak, and some other species of trees in winter ; and 
this enquiry appeared interesting, because it has long been 
well known, that very considerable chemical changes take 
place in the sap of trees in the spring ; and that the alburnum 
of trees, which are not felled before that period, is brittle, 
and subject to decay very rapidly. The heart wood of win- 
ter-felled oak trees was also formerly very generally sup- 
posed to be much more valuable than that of trees of the 
same species when felled in the spring ; and I expected to 
obtain some evidence in support of this opinion : for if the 
same fluid pervade alike the heart wood and alburnum of 
trees, it does not appear probable that the qualities of the one 
will remain unchanged, when those of the other sustain so 
much injury. 
