HEAT. 43 



in different degrees, according as its direction is parallel or 

 perpendicular to the axis of a crystal. 



If we select a substance of a different but also of a definite 

 structure, such as wood, we find that heat progresses through 

 it with more or less rapidity, according to its direction with 

 reference to the fibre of the wood : thus Decandolle and De 

 la Rive found that the conduction was better in a direction 

 parallel to the fibre than in one transverse to it ; and Dr. 

 Tyndall has added the fact, that the conduction is better in a 

 direction transverse to the fibre and layers of the wood than 

 when transverse to the fibre but parallel to the layers, though 

 in both these directions the conduction is inferior to that 

 following the direction of the fibre. Thus, in the three possible 

 directions in which the structure of wood may be contemplated, 

 we have three different degrees of progression for heat. 



In the above examples we see, as we shall see farther on 

 with reference to all the so-called imponderables, that the 

 phenomena depend upon the molecular structure of the matter 

 affected ; and although these facts are not absolutely incon- 

 sistent with the theory which supposes them to be fluids or 

 entities, it will, I think, be found to be far more consistent 

 with that which views them as motion. Heat, which we are 

 at present considering, cannot be insulated : we cannot remove 

 the heat from a substance and retain it as heat ; we can only 

 transmit it to another substance, either as heat or as some 

 other mode of force. We only know certain changes of 

 matter, for which changes heat is a generic name ; the thing 

 heat is unknown. 



Heat having been shown to be a force capable of producing 

 motion, and motion to be capable of producing the other modes 

 of force, it necessarily follows that heat is capable, mediately, 

 of producing them : I will, therefore, content myself with 

 enquiring how far heat is capable of immediately producing 

 the other modes of force. It will immediately produce 

 electricity, as shown in the beautiful experiments of Seebeck, 

 one of which I have already cited, which experiments proved, 

 that when dissimilar metals are made to touch, or are soldered 

 together, and heated at the point of contact, a current of 

 electricity flows through the metals having a definite direction 



