632 Lord Kelvin [May 21, 



de la surface metallique, modifications qui mettent un certain temps a 

 Be produire." The smallest pressures for which Pellat made his ex- 

 periments were from 3 to 4 or 5 cm. of mercury.* 



§ 14. The same method was used by Mr. J. T. Bottomley in an 

 investigation by which he demonstrated with minute accuracy the 

 equality of the Volta-contact-difference measured in a glass tube 

 exhausted to less than ^^^3 mm. of mercury* (2^ millionths of 

 an atmosphere), and immediately after in the same tube filled with 

 air to ordinary atmospheric pressure; and again exhausted and 

 filled with hydrogen to atmospheric pressure three times in succes- 

 sion ; and again exhausted and filled to atmospheric pressure with 

 oxygen. In some cases the electrical test was repeated several times, 

 while the gas was entering slowly. The actual apparatus which he 

 used is before you, and in it I think you will see with interest the 

 little Volta-condenser, with plates of zinc and copper a little larger 

 than a shilling, the upper hung on a spiral wire by a long hook 

 carrying also a small globe of soft iron. Thus you see by aid of an 

 external magnet I can lift and lower the upper plate without moving 

 the vacuum tube which, during the experiments, was kept in connec- 

 tion with a Sprengel pump and phosphoric acid drying tubes. Mr, 

 Bottomley sums up thus : " The result of my investigation, so far as 

 it has gone, is that the Volta contact effect, so long as the plates are 

 clean, is exactly the same in common air, in a high vacuum, in 

 hydrogen at small and full pressure, and in oxygen. My apparatus, 

 and the method of working during these experiments, was so sensitive 

 that I should certainly have detected a variation of 1 per cent, in 

 the value of the Volta contact efiect, if such a variation had presented 



itself." t 



§ 15. With the same method further researches have been carried 

 on by Mr. Erskine Murray, and important and interesting results 

 obtained, within the last four years, in the Physical Laboratories of 

 the Universities of Glasgow and Cambridge. He promises a paper 

 for early communication to the Eoyal Society, and, from a partial 

 copy of it which he has already given me, I am able to tell you of 

 some of his results. Taking generally as standard a gilt brass disc 

 which he found among the apparatus remaining from my experiments 

 of 1859-61, he measured Volta-differences from it in terms of the 

 modern standard one volt. These differences are what we may call 

 the Volta-potentials of the different metallic surfaces, or surfaces of 

 metallic oxides, iodides, &c., or metallic surfaces altered by cohesion 

 to them of gases or vapours, or residues of liquids which had been 

 used for washing them ; if for simplicity we agree to call the Volta- 

 potential of the gold, zero. As a rule he began each experiment by 



* A very high exhaustion had been maintained for two days, and finally per- 

 fected by two and a half hours' working at the pump immediately before the 

 electric testing experiment. 



t Brit. Assoc. Keport, 1885, pp. 901-3. 



