Papers on Contact Elect7*icity. 49 



used, in which the ordinary water between the zinc and copper 

 plates is replaced by glass, paraffin-wax, mica, shell-lac, &c.*, 

 or by dry metallic salts, such as have recently been employed 

 by Prof. Hoorweg. Mr. Brown's experiments, however, must 

 not be regarded as confuting PfafFs work of 1829. 



V. We are therefore reluctantly compelled to conclude that 

 Prof. Exner's experiments described in his first paper are in- 

 accurate, and that their striking agreement with his assump- 

 tion is due to a fortuitous combination of errors. And it may 

 be observed in passing, that the same kind of conclusion has 

 been arrived at by Prof. Young, of Princeton, regarding the 

 result communicated to the Vienna Academy of Sciences, that 

 thermoelectricity is also due to oxidation, since he has recently 

 shownf experimentally that the thermoelectric power of metals 

 is the same in air at a millionth of an atmosphere as at ordi- 

 nary pressures. 



We may mention that, for the purpose of testing whether a 

 gas has any important effect at all in contact action, we have 

 for some time been arranging apparatus to try Volta's original 

 experiments in a Crookes's vacuum. 



VI. The succeeding paper of Prof. Exner's was given in 

 the last December number of the Transactions of the Vienna 

 Academy of Sciences, and again in No. 6, B. x. H. 2, pp. 265- 

 284, of Wiedemann's Annalen. The ideas contained in it are, 

 we think, based on some misconception of the contact theory 

 of electricity. It has long been known that when the dilute 

 acid of a simple cell is saturated with oxygen there is a pri- 

 mary great electromotive force, which diminishes as the evolved 

 hydrogen begins to collect on the negative plate, not finding 

 any more free oxygen to combine with. It has also been 

 known that, if the metal from either plate is carried over and 

 deposited on the other, the electromotive force of the cell goes 

 down. Prof. Exner, however, sets himself to make careful 

 experiments with a Smee's cell, in which deposition of zinc on 

 the negative plate is rendered impossible by having the plates 

 in separate vessels connected with a glass tube drawn out to a 

 fine point; and he found that the primary electromotive force 

 was 1*15 that of a Daniell's cell, and that it diminished 

 after a current produced by short-circuiting had deposited 

 hydrogen on the platinum plate — also that, whatever was the 

 negative plate, there was the same working electromotive 

 force. u The preceding results," says Prof. Exner, " connect 



* " Contact Theory of Voltaic Action," Part II., by W. E. Ayrton and 

 John Perry, Proc. Roy. Soc. No. 106, pp. 26-34, 1878. 



t " On .the Thermoelectric Power of Iron and Platinum in vacnof by 

 Prof. E. A. Young, Phil. Mag. vol. x. p. 450 (December 1880). 



Phil. Mag. S. 5. Vol. 11. No. 65. Jan. 1881. E 



