Contact Theory of Voltaic Action. 221 



potentials is produced between the liquid and the metal. If 

 zinc is plunged in water the zinc becomes negative, the water 

 positive. Copper plunged in water also becomes negative, but 

 much less so than zinc. If two metals be plunged in water 

 (as copper and zinc), the copper, the zinc, and the water form- 

 ing a galvanic cell, all remain at one potential, and no charge 

 is observed in any part of the system." Consequently in 1875 

 we discarded our original proposed method of experimenting, 

 which was to use an apparatus somewhat similar to that em- 

 ployed by Professor Clifton, as far as we can understand it 

 without a drawing ; and we constructed the apparatus described 

 in our paper, which enabled us to measure any single contact 

 difference of potential, whether of a metal w T ith a metal, or a 

 metal with a liquid, or a liquid with a liquid, or a combination 

 of any two or more contacts. 



The very important fact that the rise of the difference of po- 

 tentials between the plates of a voltaic cell on first immersion, 

 when the circuit remains open, is due to the same cause as po- 

 larization of the plates when the circuit is closed but operating 

 in the opposite direction, as explained by Professor Clifton, 

 was clearly stated by us in our paper in question under " the 

 three states of a cell ;" and our subsequent papers showed that 

 we considered this effect to be analogous with the so-called 

 soaking in and soaking out in any dielectric, or what is called 

 the residual charge in a Leyden jar — a subject to which we 

 have been since devoting much attention. But we even went 

 further ; for we found that even when the circuit was closed 

 directly after immersion, there was first a rise of difference of 

 potentials, followed afterwards by a fall ; and this is an expla- 

 nation of a want of constancy observed in many cells, and 

 notably in the two-fluid cell described by Professor Clifton, 

 page 309. 



We take the liberty of observing that although a table of 

 the difference of potentials of the terminals of different cells is 

 of great value to practical men, still we should hardly have 

 expected to find such a table at the end of Professor Clifton's 

 paper with one number only (almost without exception) given 

 for each cell, since he was quite aware that the difference of 

 potentials between the electrodes alters from the first instant 

 of immersion of the plates. Again, we do not understand 

 how he can say that no current has passed ; for it is evident 

 that a current may pass without the electrodes being externally 

 connected. A table such as is given by Professor Clifton 

 would be very valuable if it gave the difference of potentials 

 between the electrodes when the plates had been kept im- 

 mersed for a sufficient length of time for the difference of po- 



