1897.] on Contact Electricity of Metals. 645 



immediately generated, and the discs will rise 790° in temperature, 

 and we have a column of hot brass — perhaps solid, perhaps liquid. 

 This last statement assumes that the law of electric action, on which 

 the table is founded, holds for discs 10~^ of a centimetre thick, with 

 ether or air spaces between them of 10~^ of a centimetre. In reality 

 it is probable that the law of electric action for discs 10~^ of a 

 centimetre thick, begins to merge into more complicated results of 

 intermolecular forces, before the distance is as small as 10"* of a 

 centimetre. 



Resuming our mental molecular microscopic binocular (§ 16, foot- 

 note), we cannot avoid seeing molecular structures beginning to be 

 perceptible at distances of the hundred-millionth of a centimetre, and 

 we may consider it as highly probable that the distance from any 

 point in a molecule of copper or zinc to the nearest corresponding 

 point of another 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 ether," 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 ether and dry or moist 

 atmospheric air or any gas or vapour which does not violently attack 

 the metal : or by ether 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 demonstrated 

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

 needed further foundation ; though of course I have not considered 

 quite needless or uninstructive, Pfaffs 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 2J 

 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 Hum- 

 phry Davy, who had had contemporaneous knowledge of Yolta's 

 first and second discoveries, expressed himself thus clearly as to the 

 validity of the second : " Before the experiments of M. Volta 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 neces- 

 sarily be concerned, for it was not possible to refer the electricity 

 exhibited by the opposition of metallic surfaces to any chemical 

 alterations, particularly as the effect is more distinct in a dry atmo- 



* Phil. Trans. 1807. 



