112 Lord Kelvin on 



a u 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 integration-constant as zero) can be measured 

 " thermoelectrically f and is about J millivolt at 10° C. 



* ; A voltaic force, more than a thousand times larger f, 

 " exists at the junction of the metals with the medium sur- 

 " rounding 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. 



>-> 



u References : — 



" Phil. Mag. [5] : 



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



u 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 efficacy of the supposed oxygen 

 bath in the experiments of § 2 above with varnished plates of 

 zinc and copper ? or in Erskine Murray's experiment, de- 

 scribed in his paper communicated lasL August to the Royal 

 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 

 PfafFs 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 experi- 

 ment, shown at the commencement of my lecture (§ 1 above), 



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

 t See § 24 above. K, Feb. 14, 1898, 



