478 



KKI'OKT — 1884. 



that there was no Volta effect in the slightly rarefied air then known a- 

 ' vacuum.' 



In recent times Pcllat has investigated the subject, and has eoLao to a 

 conclusion in sigreement with Pfati", viz., that the ditlereiiccs are very small. 

 The niotals used by Pellat were copper and zinc, and the gases were aii-, 

 dry and damp, dry oxygen, dry nitrogei^ dry and pure hydrogen, dry and 

 pure carbonic acid. He finds slight variations \ but exceedingly slight, 

 and such as Pt'all', lOxner, and liroun could haidly have detected. He 

 says . ' Au sui'phis, il est foi't probable (jue, si quelques-uns des auteur.s 

 precedents avaient tente les expi-riences quej'ai faites au snjet des giiz. 

 ils anraient trouve des resulfats negatifs ; les I'aibles variations produites 

 par le e.iangement des pr-oprietes du gaz quo j'ai pu mettro ni'ttcmcnt 

 en evidence, grace a la precision des mesures, sont au-dessons des erreins 

 experinientales de leurs methodes, ou a peine .superienres dans les cas les 

 plus favorables.' 



In all the above gases he has also stndied the effect of varying the 

 pressure. Lowering the pi-essure sVi'r\\i]y {iicredses the observed difference 

 of potential, but the change lags a little behind the pressure variidion. 

 Uanip and dry air l)eliave in the same wny. In oxygen the elferts ol 

 pressure arc rather better marked. Nitrogen gives nearly the siuif 

 numbers as air, but after it has been in for some time tlie numbers ai'» 

 slightly lower than at lirst. Jlydrogen gives a little greater effect than 

 even rarefied air ; rarefying hyiU'ogeu does not alter it much. Cai-boiiic 

 acid gives the same numbers as rare air or dense hydrogen. As for li([ui(ls: 

 plates wetted with alcohol give the same result as if immersed in ])lain air. 



Von Zahn ' also tried a condenser in various gases and found no dif- 

 ference, but when he tried a platinum zinc condenser in the highly 

 rarefied air now known as viicuum, with some melted .sodium in a brancl; 

 tube to absoi'b all the oxygon, the Volta effect wa.s diminished, and only 

 represented a potential difference of half a Daniell. I am not clear wlietlui 

 sodium can be trusted to ultimately absorb every trace of oxygen. I)nt i 

 should judge it would take a very long time ; and as to rarefaction- 

 dividing the numbers of molecules in a vessel by a million or two leaves 

 them quite numerous enough to .accomplish anything they want. 



Sir W. Thomson has also made experiments in different ga«cs wiils 

 negative results.- These expei'imcnts are not described in detail, l)ut 

 they were made with the apparatus shown in fig. ]'•. 



The views of Ayrton and Perry ou the subject of the effects nf 

 atmosphere nnderwt^nt modification between their first ])aper and tlnsr 

 third. \)\ their first paper they say they have good reason to believe tii:it 

 there is no great difl'erence of potential between a metallic or li(|',iii'i 

 surface and the air in contact with it. 



Clerk ]\[axwell in a letter to the ' Electrician ' •' pokes fun at tlieiii 

 for this, saying: 'A statement like this, coming from men who>i' 

 scientific energy is threatening to displace the centre of electrioil 

 development, and to carry it (juite out of Europe and America to a 

 point n uch nearer Japan, is worthy of all attention, even Avithont an 

 explicit statement of their "good reason." But iMr. J. E^own has sho^vii 

 (' Phil. Mag.' August 18'8) by the divided ring method of Sir A^ ■ 



' ]\Ieraoir quoted below. 



-■ Tliomson : IJrit. Assoc, 'J'rfins. of Srct.i., Swans-ea, 1880, p. liU. ^cc a prcviju.- 

 footnote. 



» Electrician, April '2G, H-l'.K 



