1886.] 



A Theory of Voltaic Action. 



303 



that have not an action differing from that of the air (still present to 

 a greater or less degree), or are merely neutral, there is no great 

 difference in the electrical effects produced (Pellat, ' Theses pre- 

 sentees a la Faculte des Sciences de Paris,' p. 109 ; Schulze-Berge, 

 4 Wiedemann, Annalen,' vol. 12, 1881, p. 293). The effect of a hydro- 

 gen atmosphere in gradually decreasing the negative potential near 

 platinum is an exception, and has been already well explained by the 

 hypothesis that it forms an alloy with the platinum of a more positive 

 character ; or which, according to the chemical theory,, has a greater 

 affinity for the oxygen still present than the platinum has. 



19. Many attempts have been made to remove completely from the 

 metals under examination all oxidising or other chemically active 

 substances, and thus show that the metals per se have no power of 

 producing a difference of potential. 



Some of the older writers claim to have effected this by thickly 

 varnishing the plates of the copper-zinc Volta condenser. I doubt if 

 even a thick coat of varnish, which is more or less pervious to gaseous 

 matter, would so effectually protect the surfaces as to leave no effect 

 discernible by the more sensitive modern appliances. Besides, the 

 varnish itself is objected to as a new element in the chain of " con- 

 tact" effects. Some experiments on the effect of varnish were made 

 with the large Volta condenser shown at fig. 3. 



20. This instrument was in general form copied from that described 

 by Schulze-Berge ('Wiedemann, Annalen,' vol, 12, 1881, p. 294). 

 Referring to fig. 3, A is a triangular piece of mahogany 1 inch thick, 

 turning on pointed screws at B, and resting at its other end on a 

 support M. It carries on its lower surface three vulcanite fittings, DD, 

 supporting the copper plate C, which can thus be raised from or 

 lowered towards the zinc plate Z, lying on three vulcanite levelling 

 screws, SS. The plates were 8 inches diameter, and about \ inch thick. 



A measurement of the difference of potential of the films on these 

 plates was made by a method similar to that employed by Schulze- 

 Berge and shown at fig. 4, where D is a Daniell's cell, of small 

 internal resistance, connected to RR', a set of resistance coils, of which 

 the part a is 200 ohms and a? is variable. The key K breaks the 

 connexion of both the condenser plates Cu, Zn at once. In making 

 an experiment the resistance x was adjusted till on breaking the 

 contacts at K, and immediately afterwards separating the plates, 

 no sensible deflection was observed at the quadrant electrometer E in 

 connexion with them. Then the amount of resistance in * divided by 

 the whole resistance RR' gave in terms of the Daniell the difference 

 of potential required to neutralise that of the films. 



21. The zinc plate of this condenser was carefully varnished all 

 over while hot with repeated coats of " silver lacquer " (shell-lac and 

 other gums dissolved in spirits of wine) 1 , but the difference of 



