122 Dr. Tyndall on Molecular Influences. 



dium, effected by the current or the magnet. Senarmont's* 

 and Knoblauch's} experiments demonstrate the influence of cry- 

 stalline structure upon the transmission of heat ; and the mag- 

 necrystallic discoveries of Plucker and Faraday receive, I believe, 

 their true explanation by reference simply to the modification of 

 the magnetic and diamagnetic forces which peculiarity of aggre- 

 gation induces. Matter, in this aspect, may be regarded as a 

 kind of organ through which force addresses our senses ; if the 

 organ be changed, it is reasonable to infer that the utterance 

 will be correspondingly modified, — an inference which is abun- 

 dantly corroborated by experiment. Thus, mechanical pressure 

 will polarize a ray, and the same may be applied with success to 

 produce all the phenomena of magnecrystallic action. The 

 anomalies which owe their origin to peculiarities of aggregation 

 are indeed manifold, 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 her special ends, has arranged her materials 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 J, 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 applied to metals by M. Depretz§. 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 until a 

 stationary temperature was assumed. The temperatures attained 

 by the bar, at various distances from its heated end, were ascer- 

 tained by means of thermometers fitting into cavities made to 

 receive them ; from these data, with the aid of a well-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 delicate for an inquiry like the present. Some 

 other mode must therefore be devised. I will not trouble the 



* Annates de Chimie et de Physique, vols. xxi. xxii. xxiii. 



t PoggendorfPs Annalen, vol. lxxxv. p. 169. 



X Mem. de la Soc. de Geneve, vol. iv. p. 70. 



§ Annates de Chimie et de Physique, December 1827. 



