44 CORRELATION OF PHYSICAL FORCES. 



according to the metals employed, which current continues as 

 long as an increasing temperature is gradually pervading the 

 metals, ceases when the temperature is stationary, and flows 

 in the contrary direction with the decrement of temperature. 



Another class of phenomena which have been generally 

 attributed to the effects of radiant heat, and to which, from 

 this belief, the term thermography has been applied, may also, 

 in their turn, be made to exhibit electrical effects effects 

 here of Franklinic or static electricity, Seebeck's experiments 

 showing effects of voltaic or dynamic electricity. 



If polished discs of dissimilar metals say, zinc and copper 

 -be brought into close proximity, and kept there for some 

 time, and either of them has irregularities upon its surface, a 

 superficial outline of these irregularities is traceable upon the 

 other disc, and vice versa. Many theories have been framed 

 to account for this phenomenon, but whether it be due or not 

 to thermic radiations, the relative temperature of the discs, 

 their relative capacities and conducting and radiating powers 

 for heat, undoubtedly influence the phenomena. 



Now, if two such discs in close proximity be connected 

 with a delicate electroscope, and then suddenly separated, the 

 electroscope is affected, showing that the reciprocal radiation 

 from surface to surface has produced electrical force. I cite 

 this experiment in treating of heat as an initial force, because 

 at present the probabilities are in favour of thermic radiation 

 producing the phenomenon. The origin of these so-called 

 thermographic effects is, however, a question open to much 

 doubt, and needs much further experiment. When I first 

 published the experiment which showed that the mere 

 approximation of discs of dissimilar metals would give rise to 

 electrical effects, I mentioned that I considered the fact of 

 the superficial change upon the surface of metals in proximity, 

 and, a fortiori, in contact, would explain the development of 

 electricity in Volta's original contact experiment, without 

 having recourse to the contact theory, i.e. a theory which 

 supposes a force to be produced by mere contact of dissimilar 

 metals without any molecular or chemical change. I have 

 seen nothing to alter this view. Mr. Gassiot has repeated 

 and verified my experiment with more delicate apparatus and 



