'290 



Prof. Franz Exner on Contact Electricity. 



Since the heat of combustion of iron is 34,100 calories, cal- 

 culation gives for Fe | Pt the value '701 Daniell, which agrees 

 perfectly with the observed value. 



Finally silver was submitted to experiment ; but, on account 

 of its small heat of oxidation, its difference of potentialwith 

 platinum is so small that the smallest error of observation mo- 

 difies considerably the result. I have therefore examined 

 silver only in order to be able to make a direct comparison 

 with the experiment given further on relating to silver in a 

 chlorine atmosphere. For the value Ag | Pt in air I have 

 only made one measurement, which, on account of the said com- 

 parison, may be given here. The plate used Was a solid silver 

 plate ; for galvanoplastic silver films possess a permeability for 

 air in a yet higher degree than gold films. 

 The notation being as before, we get : — 



A. 



B. 



C. 



D. 



E. 



P. 



a. 



&N-800 

 a=855 



N=800 

 a =735 



D=60 



N=800 

 a =705 



x=5 



x=5 



#=•083 



Since, according to Thomsen, the heat of combustion of 

 silver is 3000 calories, the value Ag | Pt comes out "062 

 Daniell, which, taking into account the smallness of the value, 

 corresponds sufficiently well with the experiment. 



I now tried the experiment of surrounding the two metals 

 of the condenser with different gases ; and for this purpose 

 I chose both plates of the same metal, viz. silver, allowing 

 air to act on one, but chlorine gas on the other. For this ex- 

 periment the apparatus must be arranged as follows : — A short 

 cylindrical glass tube was closed air-tight at one end with one 

 of the silver plates, but so that the plate did not touch the 

 glass anywhere, a condition easily attained by cementing 

 with paraffin. The lower end of the vertically placed glass 

 tube was closed air-tight by a stopper, through which passed 

 two small glass tubes for the entrance and exit of the gas, 

 and also a platinum wire thoroughly insulated by paraffin, 

 the inner end of which pressed against the silver plate. This 

 latter was merely for the purpose of making the metallic 

 connexion of the condenser. The silver plate was now co- 

 vered on its outer surface with paraffin, which formed the 

 insulating layer of the condenser. 



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

 could be placed on this condenser exactly as in the earlier 

 experiments. 



If now the condenser-plates were connected, there was of 

 course not the slightest charge apparent. As soon, however, 



