288 Prof. Franz Exner on Contact Electricity. 



In each of the experiments 11 and 12 a check observation 



is omitted. The experiments 10 to 13 refer in reality to 

 zinc and gold, as in them the platinum plate was replaced by 

 a thickly-gilded brass one. It is evident that gold and plati- 

 num behave quite similarly. I must, however, here remark 

 that this equality did not continue long. Even after two or 

 three days the gold plate gave considerably smaller values 

 than the platinum ; and it is not long till such a gilt brass 

 plate behaves almost the same as one without gilding. This 

 can only be due to the fact that the air gradually permeates 

 the gold, and oxidizes the brass underneath. 



A similar observation has been made by De La Hive with 

 varnished plates, so long as the varnish layer has not a very 

 considerable thickness. This behaviour of a thin metallic cover- 

 ing is of interest, because the most apparently trustworthy of 

 the old experiments were made with such gilt and platinized 

 plates, for example those of Kohlrausch. 



This author arranged special experiments * to try if a 

 platinized brass plate behaved the same as a thick platinum 

 one, and found that it did so. Still this experiment was in 

 all probability tried soon after the platinizing; and I think I 

 do not err in ascribing the smallness of the value for Zn|Pt 

 which Kohlrausch obtained, viz. *6 Daniell, instead of '88, to 

 a gradual permeation of air through the platinum covering. 



This is confirmed by the fact that Kohlrausch found the ratio 



very variable, and also (in comparison with the 



observations given below) too large, viz. — — — — to — — . If 



the observed value Zn|Pt was too small, this ratio would 

 evidently be too great. 



I could not find that any other author besides Kohlrausch 

 had given any data on the absolute value of Zn | Pt. 



In order now to obtain an idea of the correctness of this 

 value, or perhaps of the chemical theory itself, we have only 

 to divide half the value of the heat of oxidation of zinc by the 

 heat-value of the Daniell cell. According to J. Thomsen the 

 heat of oxidation of zinc is 42,700 calories per equivalent ; 

 and the heat- value of the Daniell cell is similarly calculated 

 from Thomsen's figures equal 24,300 calories. From these 

 the value of Zn|Pt is found to be "879 Daniell. This number 

 corresponds with observations better than could have been 

 expected with such difficult experiments. 



* Pogg. Ann, lxxxii. p. 407, 1851. 



