CHEMICAL DECOMPOSITION. 241 



A question presents itself here in reference to Peltier's phenomenon. 

 The thermal effect observed during the passage of the current at the 

 soldering of the two metals, measures the sudden fall of potential at 

 this point, and it would seem as if it should measure the electro- 

 motive force of contact between them on Yalta's theory. Does the 

 result thus obtained agree with that given by other methods the use 

 of electrometers, for instance ? 



Experiment answers this question in the negative ; not merely do 

 the series of numbers obtained by the two methods disagree with 

 each other, but the bodies themselves are not arranged in the same 

 order ; the numbers of the two series are not of the same order of 

 magnitude ; they are even sometimes of opposite signs. It is certain 

 therefore that we are not measuring the same phenomenon in the 

 two cases. The most plausible explanation of this discrepancy is that, 

 in the electrostatic measurements, we are dealing with a complicated 

 phenomenon in which the nature of the medium, necessarily inter- 

 posed between the metals in contact, plays a considerable part. 



249. CHEMICAL DECOMPOSITION. Whenever a compound 

 liquid is traversed by a current it splits up ; one of the elements 

 appears at the conductor by which the current arrives, the other at 

 that by which it leaves. Faraday gave to this phenomenon the 

 name electrolysis ; the body submitted to decomposition he called an 

 electrolyte, and applied the term electrodes to the two conductors by 

 which the current enters and leaves ; the former being the positive 

 electrode, and the latter the negative electrode.* 



Two conditions are necessary for the occurrence of electrolysis ; 

 the current must traverse the compound, and the compound itself 

 must be liquid, or at any rate in the pasty state. Thus, glass at a 

 red heat gives evident signs of decomposition, for it becomes at 

 once a conductor, and pasty. 



It is extremely remarkable that the products of decomposition 

 only appear on the electrodes. Clausius, developing a theory which 

 was originally propounded by Grotthiis, explains this phenomenon 

 in a very ingenious manner. On his view the molecules of which 

 the body is made up are in a constant state of agitation ; but while 

 the excursions of each molecule are restricted in the case of solids, 

 these excursions may take place to any extent and in any directions 

 in liquids. Thus the molecules of hydrogen which form part of the 



* Faraday called the electrode by which the current enters the anode, and that by 

 which it leaves the cathode ; he applied the term ions to the elements decomposed. 

 The anion is that which is liberated on the anode, the cation that on the cathode; 

 these terms have not however, like the former, been generally adopted 



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