478 report — 1884. 



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

 ' vacuum.' 



In recent times Pellat has investigated the subject, and has come to a 

 conclusion in agreement with Pfaff, viz., that the differences are very small. 

 The metals used by Pellat were copper and zinc, and the gases were air, 

 dry and damp, dry oxygen, dry nitrogen, dry and pure hydrogen, dry and 

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

 and such as Pfaff, Exner, and Brown could hardly have detected. He 

 says : ' Au surplus, il est fort probable que, si quelques-uns des auteura 

 precedents avaient tente les experiences que j'ai faites au sujet des gaz, 

 ils aui'aient trouve des resultats negatifs ; les faibles variations produites 

 par le changement des prcprietes du gaz que j'ai pu mettre nettement 

 en evidence, grace a la precision des mesures, sont au-dessous des erreurs 

 experimentales de leurs methodes, ou a peine superieures dans les cas les 

 plus favorables.' 



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

 pressure. Lowering the pressure slightly increases the observed difference 

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

 Damp and dry air behave in the same way. In oxygen the effects of 

 pressure are rather better marked. Nitrogen gives nearly the same 

 numbers as air, but after it has been in for some time the numbers are 

 slightly lower than at first. Hydrogen gives a little greater effect than 

 even rarefied air ; rarefying hydrogen does not alter it much. Carbonic 

 acid gives the same numbers as rare air or dense hydrogen. As for liquids : 

 plates wetted with alcohol give the same result as if immersed in plain air. 



Von Zahn l 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 vacuum, with some melted sodium in a branch 

 tube to absorb all the oxygen, the Volta effect was diminished, and only 

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

 sodium can be trusted to ultimately absorb every trace of oxygen, but I 

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

 dividing the numbers of molecules iu 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 gases with 

 negative results. 2 These experiments are not described in detail, but 

 they were made with the apparatus shown in fig. 10. 



The views of Ayrton and Perry on the subject of the effects of 

 atmosphere underwent modification between their first paper and their 

 third. In their first paper they say they have good reason to believe that 

 there is no great difference of potential between a metallic or liquid 

 surface and the air in contact with it. 



Clerk Maxwell in a letter to the ' Electrician ' 3 pokes fun at them 

 for this, saying: 'A statement like this, coming from men whose 

 scientific energy is threatening to displace the centre of electrical 

 development, and to carry it quite out of Europe and America to a 

 point much nearer Japan, is worthy of all attention, even without an 

 explicit statement of their " good reason." But Mr. J. Brown has shown 

 (' Phil. Mag.' August 1878) by the divided ring method of Sir W. 



1 Memoir quoted below. 



- Thomson : Brit. Assoc., Trans, of Sects., Swansea, 18S0, p. 40-1. See a previous 

 footnote. 



3 Electrician, April 20, 1S79. 



