272 Mr. Knight on the extent of the 
The opposite sides of the same transverse sections of wood 
were divided by the saw in a direction diametrically opposite 
to that above mentioned ; under which circumstances, the 
expansion of the convergent cellular processes could not, as in 
the preceding cases, occasion any pressure upon the sides of 
the saw, which consequently continued to move with perfect 
freedom. 
These circumstances led me to infer, that the medullary 
canal must be subject to considerable variations of diameter, 
with the increase or diminution of the quantity of moisture in 
the wood ; and I conceived, that I should easily be able to 
ascertain the truth or falsehood of this conjecture by the fol- 
lowing means. I selected, in winter, some parts of the stems 
of young trees as soon as they were felled, which I retained 
in such a situation as might occasion them to lose a consider- 
able part of the water they contained, though not to such an 
extent as to destroy, or endanger, life. The medulla of these 
was then removed ; and the space it had occupied was filled 
with cylindrical pieces of metal, which were so large that 
they could not be introduced without considerable force. The 
pieces of wood were then deposited in a damp soil, from 
which they absorbed much moisture ; and at the distance of 
ten days, I found the medullary canal so much enlarged, that 
the pieces of metal dropped through without any pressure 
being applied. 
I am prepared to prove, in a future communication, that 
the quantity of moisture in the alburnum is subject to great 
variations in the living tree, and therefore I conclude, that - 
the medullary canal frequently changes the extent of its 
diameter. 
