754 THE POPULAR SCIENCE MONTHLY. 





 Faraday of the electro-chemical theory these are facts fresh in all 



our memories. The justice of time, however, in this case, if it has been 

 tardy, has been none the less sure. The experiments of Thomson have 

 vindicated Volta and established the contact theory as a vera causa. 

 And, more curiously still, it now appears to be proved that both con- 

 tact and chemical action underlie the production of that very animal 

 electricity so stoutly battled for by Galvani and his associates. 



Volta's experiments to prove that a difference of potential is de- 

 veloped by the contact of two hetei'ogeneous metals were not crucial. 

 But Thomson, repeating them with the aid of more delicate apparatus, 

 has shown that, whenever copper and zinc are brought in contact, the 

 copper becomes negative to the zinc. In proof that the chemical action 

 of atmospheric moisture was not the cause of the phenomenon, he 

 showed that, when a drop of water served to connect the copper and 

 the zinc, no charge at all was produced. The fact may therefore be 

 regarded as established, as the result of numerous and varied experi- 

 ments, that a difference of electrical potential is always developed at 

 the surfaces of contact of heterogeneous media. Not only is this true 

 of solids in contact with solids, but also of solids in contact with 

 liquids, and of liquids in contact with each other. Of course, the pro- 

 duction of electricit} r by contact must result from a loss of energy else- 

 where. In the opinion of Cumming, it is the loss of energy which is 

 owing to the unsymmetrical swinging of the molecules on the two sides 

 of the surfaces of contact, which reappears as difference of potential 

 between the solids or as the energy of electrical separation. 



But we may carry the sequence yet another step backward. The 

 energy which is thus lost at the surfaces of separation must be heat, 

 and this junction must be cooled thereby. Thus the production of 

 thermo-electricity is seen only to be a special case of a general law, a 

 view to which the well-known Peltier effect gives support. In this 

 phenomenon, when two metals are joined together in the form of a 

 ring and one junction is heated, a current is produced which cools the 

 other junction. From a study of these conditions, Thomson has con- 

 cluded that the absorption of heat in a thermo-electric circuit varies 

 for different metals with the direction of the current. Thus in iron, 

 the current from hot to cold absorbs heat, while in copper the current 

 which absorbs heat is from cold to hot. In entire accordance with 

 these results are the conclusions recently reached by Hoorweg. When- 

 ever two conductors come into contact, motion of heat results in the 

 development of electricity, the current produced existing at the cost 

 of heat at one part of the point of contact, and evolving heat at the 

 other for a result. Hence all voltaic currents are thermo-currents. , 



To return to the muscle, it must now be apparent that the electrical 

 charge which appears in its fiber may have its origin in so purely a 

 physical cause as the contact of the heterogeneous substances of which 

 the tissue is built up ; the maintenance of this charge being effected 



