482 keport — 1884. 



suited in the conclusion that a condenser made of two different metals 

 showed nearly the same Volta effect, whether the atmosphere surrounding 

 the plates was air or hydrogen. 



If it be assumed that the experiments of Brown and Schultze-Berge 

 establish their point, and that Pellat's apparatus for different gases (fig. 9) 

 is satisfactory (rather a large assumption), I am unable to reconcile the 

 discrepancy, except by suggesting that Pellat did not take sufficient pains 

 to remove the condensed air sheet originally on both his plates. It is of 

 course just possible that the difference between the potentials of the twi > 

 metals might be the same in two gases though the absolute potential of 

 both was different, but it is improbable. 



In this connection I must notice also a rather long memoir ' by Dr. W. 

 von Zahn, published in 1882, which reviews the whole subject, and de- 

 scribes an elaborate series of measurements made with an apparatus 

 something like what one might suppose Ayrton and Perry's to become if 

 it were arranged for use in different gases and in vacuo. He refers with 

 admiration to Pellat's work in the preface, and I do not suppose imagines 

 that his own numerical determinations can compare with Pellat's for 

 accuracy where they overlap, seeing that he only makes use of a sort of 

 combination of Kohlrausch's and Hankel's methods, with a Hankel elec- 

 trometer as a -measuring instrument. 2 He has tried, however, a large 

 number of substances as well as ordinary metals such as powdered 

 antimony, iron and nickel reduced by hydrogen, many kinds of carbon, 

 Pe 3 4 , manganite, pyrolusite, copper oxide, lead 'hyperoxide,' iron 

 glance, and other minerals. He has measured the Volta effect in various 

 gases and at different pressures, and finds, like Pellat, that it does not 

 appreciably vary. 



He has also examined the effect of temperature on the Volta effect,, 

 though he appears to think that this ought to bear some close relation to the 

 phenomenon of Seebeck, a natural mistake many years ago when made by 

 Avenarius, whom it led most happily though fortuitously to the true, and 

 by him experimentally verified, law of E.M.F. in a thermoelectric circuit/ 

 However, Zahn finds that experiment lends no support to this view, and 

 says that a larger series of results must be obtained before basing a theory 

 on them. Von Zahn is a confirmed contact theorist, and he victoriously 

 assails several experiments supposed to be distinctly in favour of a chemi- 

 cal view of the Volta effect. He says he publishes his results because of the 

 extraordinary discoveries being propounded by Professor Exner (such as 

 that a thermopile will not work in a vacuum), 4 and because of the vague 



1 Untersuclaaigen iibcr GmtacteleJdricitai, von Dr. W. v. Zahn, Leipzig. Tcubncr, 

 1882. 



2 A Hankel electrometer is a modification of Bohnenberger's, in which a batterj 

 with middle to earth is substituted for the dry pile; the plates on either side of the 

 ^old leaf are minutely adjustable, and the motions of the gold leaf are read by a 

 microscope. It is sometimes preferred to a quadrant for its small capacity and dead 

 quick motion ; it can be made very sensitive, but it can hardly be a satisfactory 

 measuring instrument. Pellat used it, but only as an electroscope. 



3 Avenarius: 'Die Thermoelektricitat, ihrem Ursprunge nacli, als identisch mit 

 der Contactclcktricitlit betrachtet,' Pogg. Ann. cxix. 18G3. See also Pogg. Ann. 

 exxii., where he proceeds to calculate Volta effects from thermoelectric data. 



4 I have been unable to find this extraordinary statement in Exncr's works, but 

 it is quoted again by Ayrton and Perry, Phil. Mag., 1881, p. -19. Exner seems to 

 have said that the thermoelectric power of bismuth-antimony is destroyed by 

 immersing the pile in pure nitrogen, and Young of Princetown takes the trouble to 

 examine whether it is so experimentally (see Phil. Mag. x. 1880, p. 400), and finds 



