Modes of Physical Force. 95 



direction as long as the temperature continues to rise, 

 and in the other direction in the contrary case of cool- 

 ing. Heat and light go hand in hand continually in 

 such a way as to suggest the notion that they are in- 

 separable. And certainly the modes of action of radiant 

 heat and of light are so similar, both being subject to 

 the same laws of reflection, refraction, double-refraction, 

 and polarization, that their difference appears to exist 

 more in the manner in which they affect the senses than in 

 their modes of physical action. With regard to chemical 

 affinity and magnetism, perhaps, the only method by 

 which in strictness this form of heat may be said to pro- 

 duce them is through the medium of electricity, the 

 thermo-electrical current, produced by heating dissimilar 

 metals, being capable of deflecting the magnet, of mag- 

 netizing iron, of forming and decomposing chemical 

 compounds, and that too in proportion to the amount 

 of heat. 



Whatever else it be electricity is certainly an af- 

 fection of matter for which electrolysis and polarization 

 and induction are other names. There is no reason to 

 believe that conductors, and non-conductors or dielec- 

 trics, behave differently. That a dielectric is molecu- 

 larly polarized is seen in more ways than one. One 

 way, pointed out by Matteucci, is to take a number of 

 thin plates of mica, to arrange them like a pack of 

 cards, to face the two outer plates externally with tin 

 foil, to electrify one of the facings from a friction- 

 machine, and then to separate the plates with insulating 

 handles. When this is done it is found that each plate 

 is separately electrified, one side of it being positive and 

 the other negative a result showing very conclusively 

 a polarization throughout the whole pack of plates as 

 the effect of induction. Another way pointed out by 



