48 Messrs. Ayrton and Perry on Prof. Exner's 



and silver in air, and which, as has been recently pointed out by 

 Prof. Hoorwegj proved nothing but the well-known fact that 

 silver chloride is an electrolyte. For Prof. Exner's arrange- 

 ment was this : — "A short glass cylinder was closed air-tight 

 at its upper end with a plate of silver, which did not, how- 

 ever, touch the glass, as there was a collar of paraffin-wax. 

 The bottom of the vertical glass tube was closed air-tight 

 with a cork, through which passed two small glass tubes to 

 admit the gas and allow it to escape, and also a platinum wire 

 well insulated by paraffin-wax, and the inner end of which 

 touched the silver plate. This latter w T as only for making 

 metallic connexion with the condenser. 



"A second silver plate, of the same dimensions as the 

 first, could be placed on this condenser in the same way as in 

 the earlier experiments. 



" ISTow, if the condenser-plates were connected, there was 

 naturally not the least charge. But as soon as the interior 

 of the glass tube was filled with dry chlorine, the condenser 

 showed at once a considerable and quite constant electromotive 

 force." 



Of course it did, we should reply, seeing that a galvanic cell 

 had been formed of platinum, chloride of silver, and silver. 

 If, however, the platinum wire had been soldered to the silver 

 plate, instead of merely loosely touching it, no such effect 

 would have been observed. 



Prof. Exner refers to Mr. Brown's most interesting series of 

 experiments*; but we feel that, although carefully made and 

 faithfully described, they cannot throw much light on the rela- 

 tive values of the so-called contact and chemical theories. For 

 Mr. Brown's zinc and copper plates were coated with zinc sul- 

 phide, copper sulphide, or zinc chloride and copper chloride ; 

 and every supporter of the contact theory is prepared to admit 

 that there is a difference of potential between a metal and its 

 oxide, chloride, or sulphide, or any other of its salts in a dry or 

 wet state. In fact, in Sir Wm. Thomson's earliest experiments 

 on contact, he refers to the great change produced in the mea- 

 sured difference of potentials of a point in the air close to the 

 zinc, and of a point close to the copper, if the copper be allowed 

 to oxidize f. Had Mr. Brown's apparatus enabled him to 

 make quantitative experiments instead of only qualitative ones, 

 his results would have been of great value in connexion with 

 the electromotive forces of galvanic cells such as we have 



* " Theory of Voltaic Action," by J. Brown, Phil. Mag. Aug. 1878, 

 pp. 142-145, and Feb. 1879, pp. 109-111. 



t u New Proof of Contact Electricity," by Sir Wm. Thomson, Proc. 

 Lit. and Phil. Soc. of Manchester, Jan. 21, 1862. 



