DR. TYNDALL ON MOLECULAR INFLUENCES. 
227 
across it. But here the exceptions are so numerous that we have no warranty for a 
general conclusion. 
But the most remarkable result of the experiments remains yet to be stated. If 
we cast our eyes along the second and third columns of the tabular summary, we shall 
find that in every instance the velocity of propagation is greatest in a direction per- 
pendicular to the ligneous layers. This result is, of course, wholly independent of 
the specific heat of the wood, inasmuch as it is two directions through the self-same 
cube which are here compared with each other. The law of molecular action, as 
regards the transmission of heat through wood, may therefore be expressed as fol- 
lows : — 
At all the points not situate in the centre of the tree, wood possesses three unequal axes 
of calorific conduction, which are at right angles to each other. The first, and principal 
axis, is parallel to the fibre of the wood; the second, and intermediate axis, is perpen- 
dicular to the fibre and to the ligneous layers; while the third and least axis is perpen- 
dicular to the fibre and parallel to the layers. 
The researches of Savart on the sonorous vibrations of wood naturally suggest 
themselves here ; for, doubtless, the same molecular structure which imparts to this 
substance the peculiar elastic properties discovered by Savart, must be regarded 
as the cause of the differential action established above. Savart* took bars of equal 
size, and in different directions, from a mass of wood ; determining their resistance 
to flexure from the number of vibrations carried out by each in a certain time, he 
found that wood possessed three axes of elasticity. These axes coincide with the axes 
of calorific conduction established by the foregoing experiments. The axis of greatest 
elasticity coincides with that of highest conductive capacity, and the axis of least 
elasticity with that of lowest conductive capacity. 
A few exact experiments made with a view to ascertain the influence of molecular 
structure upon the cleavage of wood would have formed an interesting addition to 
this communication; fortunately, however, the mere sense of touch, to fingers 
accustomed to seek for the cleavages of crystals, affords sufficient evidence here. If 
a piece of wood be taken, on which the rings which mark the growth of the tree 
plainly appear, and a penknife or a chisel be laid across the rings, it will be found 
that the pressure necessary to cleave the wood is less in this than in any other direc- 
tion. The cohesion in the direction parallel to the layers and perpendicular to the 
fibre is therefore a minimum. In the same way, it will be found that of all lines per- 
pendicular to the fibre the line of greatest cohesion is perpendicular to the ligneous 
layers ; while the cohesion in the direction of the fibre is far greater than along 
either of the lines just referred to. Hence — 
Wood possesses three axes of cohesion which coincide with the axes of calorfic conduc- 
tion — the greatest with the greatest, and the least with the least. 
It would have also added interest to the inquiry to have examined the permeability 
* Taylor’s Scientific Memoirs, vol. i. p. 139, 
2 H 
MDCCCLIII. 
