320 Mr. Knight on the Origin and Office of 
trees, and to be analogous, in some degree, in their effects, 
to the cavities in the bones of animals ; by which any degree 
of strength, that is necessary, is given with less expenditure 
of materials, or the incumberance of unnecessary weight; 
and the wood of many different species of trees is thus made, 
at the same time, very light, and very strong, the rigid vege- 
table fibres being placed at greater distances from each other 
by the intervention of alburnous tubes, and consequently 
acting with greater mechanical advantage, than they would if 
placed immediately in contact with each other. 
I have shewn in a former communication, that the specific 
gravity of the sap increases during its ascent in the spring, 
and that saccharine matter is generated, which did not pre- 
viously exist in the alburnum, nor in the sap, as it rose from 
the root : and I conceive it not to be improbable, that the air 
contained in the alburnous tubes may be instrumental in the 
generation of this saccharine matter. For I discovered in the 
last autumn, that much air is absorbed, or at least disappears, 
during the process of grinding apples for the purpose of 
making cider, and that during this absorption of air, the juice 
of acid apples becomes very sweet, and acquires many degrees 
of encreased specific gravity ; and a similar absorption of air, 
with corresponding effects, is well known to take place in the 
process of malting. 
1 shall conclude with observing, that in retracting the 
opinion I formerly entertained respecting the ascent of the sap 
in the alburnous tubes, I do not mean to retract any opinion 
that I have given in former communications respecting the 
subsequent motion of the sap through the central vessels, the 
leaves, and bark ; or the subsequent junction of the descend- 
