218 
DR. TYNDALL ON MOLECULAR INFLUENCES. 
applied with success to produce all the phenomena of raagnecrystallic action. The 
anomalies which owe their origin to peculiarities of aggregation are indeed manitold, 
and constitute one of the most important subjects of study which can engage the 
attention of the natural philosopher. 
Organic structures furnish an ample field for inquiries into molecular action. For 
here, as before remarked, nature, to attain ber special ends, has arranged her mate- 
rials in a particular manner. To ascertain what effect the molecular structure of wood 
has upon the transmission of heat through it, constitutes the object of the first part of 
this investigation. 
Upwards of twenty years ago MM. De la Rive and DeCandolle instituted an 
inquiry into the conductive power of wood*, and in the case of five specimens 
examined established the fact of the feeble conductivity of the substance, and also 
that the velocity of transmission was greater along the fibre than across it. The 
manner of experiment was that usually adopted in inquiries of this nature, and ap- 
plied to metals by M. DEPRExzf. A bar of the substance was taken, one end of 
which was brought into contact with a source of heat and allowed to remain so iinti 
a stationary temperature was assumed. The temperatures attained by the bar, at 
various distances from its heated end, were ascertained by means of therinometers 
fitting into cavities made to receive them ; from these data, with the aid o a we - 
known formula, the conductivity of the wood was determined. Since the publication 
of their results by the distinguished men above mentioned, nothing, so far as I am 
aware of, has been done in connexion with this subject. 
The mode of experiment here indicated is, however, by no means sufficiently deli- 
cate for an inquiry like the present. Some other .mode must therefore be devised. 
I will not trouble the reader with a rehearsal of the long series of trials which la^e 
led to the construction of the instrument employed in these researches, but will pro- 
ceed at once to the description of it. ^ , -a 
QQ' RR', fig. 1, is an oblong piece of mahogany 3-4 inches long, I'S me wi e 
and 0-5 of an inch deep. A is a bar of antimony, B is a bar of bismuth each 
measuring T5 inch in length, 0-07 of an inch in breadth and 0-3 of an inch in depth. 
The ends of the two bars are kept in close contact by the iyory jaws I, I', and the other 
ends are let into a second piece of ivory i, in which they are firmly fixed. Soldered to 
these ends are two pieces of platinum wire which proceed to the little ivory cups MM , 
enter through the sides of the cups and communicate with a drop of mercury placed m 
the interior. The wood is cut away, so that the bars A and B are sunk to a depth wliic i 
places their upper surfaces a little below the general level of the slab of mahogany. 
The ivory jaws I, T are sunk similarly. Two small projections are observed m the 
figure iutting from I, T; across from one projection to the other a fine membrane is 
drawn, thus enclosing a little chamber m, in front of the wedge-hke end ^ ^ le 
bismuth and antimony junction ; the chamber has an ivory bottom. S is a wooden 
* Mem. de la Soc. de Geneve, vol. iv. p. 70. t Annales de Chim. et de Phys. December 182/. 
