1897.] on Contact Electricity of Metals. 547 



in December, I have to-day (Feb. 14), received from him a "slightly 

 amplified statement made in order to concentrate the differences," 

 which he kindly gives me for publication as a supplement to the 

 shorter statement from the syllabus. 



Amplification, February, 1898. 



" There is a true contact-force at a zinc-copper junction,* which 

 " on a simple and natural hypothesis (equivalent to taking an inte- 

 " gration-constant as zero) can be measured thermoelectrically f and 

 " is about ^ millivolt at 10° C. 



" A voltaic force, more than a thousand times larger,f exists at 

 " the junction of the metals with the medium surrounding them ; and 

 " in an ordinary case is calculable as the difference of oxidation- 

 " energies of zinc and copper ; but it has nothing to do with the heat 

 " of formation of brass. 



" References : 



" Phil, Mag. [5]. 



" vol. xix. pp. 360 and 363, brass and atoms, pp. 487 and 494, summary. 



" vol. xxi. pp. 270 and 275, thermoelectric argument. 



'• vol. xxii. p. 71, Ostwald experiment. 



" August 1878, Brown experiment." 



§ 36. With respect to the first of the two paragraphs of this 

 last statement and the first two lines of the second, the wrongness 

 of the view there set forth is pointed out in § 24 above. With 

 respect to the last clause of the second paragraph and the statement 

 quoted from the syllabus, I would ask any reader to answer these 

 questions : — 



(i.) What would be the ef&cacy of the supposed oxygen bath in 

 the experiments of § 2 above with varnished plates of zinc and 

 copper ? or in Erskine Murray's experiment, described in his paper 

 communicated last August to the Eoyal Society, in which metallic 

 surfaces, scraped under melted paraffin so as to remove condensed 

 oxygen or nitrogen from them, and leave fresh metallic surfaces in 

 contact with a hydro-carbon, are subjected to the Voltaic experiment ? 

 or in Pfaff's and my own and Pellat's experiments with different 

 gases, at ordinary and at low pressures, substituted for air ? or in 

 Bottomley's high vacuum and hydrogen and oxygen experiments 

 (§ 14 above) ? 



(ii.) What would be the result of Volta's primary experiment, 

 shown at the commencement of my lecture (§1 above), if it had been 

 performed in some locality of the universe a thousand kilometres 

 away from any place where there is oxygen ? The insulators may 

 be supposed to be made of rock-salt or solid paraffin, so that there 

 may be no oxygen in any part of the apparatus. This I say because 

 I understand that some anti-Voltaists have explained Bottomley's 



* See footnote on § 16 above. K. Feb. 14, 1898. 

 t See § 24 above. K. Feb. 14, 1898. 



