110 Lord Kelvin on 



molecule is less than one one-hundred-millionth, and greater 

 than one one-thousand-millionth of a centimetre.] 



§ 33. In all that precedes I have, by frequent repetition of 

 the phrase "air or aether," carefully kept in view the truth 

 that the dry Volta contact-electricity of metals is, in the 

 main, independent of the character of the insulating medium 

 occupying space around and between the metals concerned 

 in each experiment, and depends essentially on the chemical 

 and physical conditions of molecules of matter in the thin 

 surface stratum between the interior homogeneous metal and 

 the external space, occupied by aether and dry or moist atmo- 

 spheric air or any gas or vapour which does not violently 

 attack the metal : or by aBther with vapours only of mercury 

 and glass and platinum and steel and vaseline (caulking the 

 glass-stopcocks), as in Bottomley's experiments (§ 14 above). 



This truth has always seemed to me convincingly demon- 

 strated by Volta's own experiments, and I have never felt 

 that that conviction needed further foundation ; though of 

 course 1 have not considered quite needless or uninstructive 

 Pfaff's and my own and Pellat's repetitions and verifications, 

 in different gases at different pressures, and Bottomley's 

 extension of the demonstration to vacuum of 2^ millionths of 

 an atmosphere. I am now much interested to see by 

 Professor Oliver Lodge's report," already referred to (§4 

 above), that in the Bakerian Lecture to the Royal Society in 

 1806 *, Sir Humphry Davy, who had had contemporaneous 

 knowledge of Volta's first and second discoveries, expressed 

 himself thus clearly as to the validity of the second : " Before 

 the experiments of M. Yolta on the electricity excited by mere 

 contact of metals were published, I had to a certain extent 

 adopted this opinion/' an opinion of Fabroni's ; " but the 

 new fact immediately proved that another power must 

 necessarily be concerned, for it was not possible to refer the 

 electricitv exhibited by the opposition of metallic surfaces to 

 any chemical alterations, particularly as the effect is more dis- 

 tinct in a dry atmosphere, in which even the most oxidizable 

 metals do not change, than in a moist one, in which many 

 metals undergo oxidation." 



§ 34. It is curious to find, thirty or forty years later, De la 

 Rive explaining away Volta's second discovery by moisture 

 in the atmosphere ! Fifty-one years ago, when I first learned 

 Volta's second discovery, by buying, in Paris, apparatus by 

 which it has ever since been shown in the ordinary lectures 

 of my class in the University of Glasgow, I was earned that 

 De la Rive had found it wrong, and had proved it to be due 

 * Phil. Trans. 1807. 



