58 CORRELATION OF PHYSICAL FORCES. 



ments 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 defi 

 nite direction according to the metals employed, which cur 

 rent continues as long as an increasing temperature is grad 

 ually pervading the metals, ceases when the temperature is 

 stationary, and flows in the contrary direction with the decre 

 ment 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 ef 

 fects here of Franklinic or static electricity, as Seebeck s ex 

 periments showed effects of voltaic or dynamic electricity. 



If polished discs of dissimilar metals say, zinc and cop 

 per be brought into close proximity, and kept there for 

 some time, and either of them has irregularities upon its sur 

 face, 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 phe 

 nomena. 



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 ra 

 diation 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 metallic discs would give rise to electrical 



