Mr. F. S. Spiers on Contact Electricity. 83 



the bottom of the surface facing the platinum. Cleaning it 

 well with emery-paper sent back the value to 



1-20 volts. 



Thus we have arrived at the exceedingly interesting fact 

 that even in a high vacuum of pure dry hydrogen at the minute 

 pressure of j-^ mm. of mercury, and after four wasldngs in 

 that gas, there is still sufficient oxygen present to completely 

 oxidize the surface of an aluminium plate, if it only be brought 

 to a sufficiently high temperature (about a dull red heat) ; 

 and if there is sufficient oxygen present to oxidize it there is 

 surely sufficient present to charge it as required by the 

 electrolytic theory o£ the Volta effect. 



In face of this striking experimental result, it is utterly 

 futile to draw any positive conclusions whatsoever concerning 

 the seat of the E.M.F. in Volta's phenomenon based on 

 experiments made in what usually passes for a vacuum, 

 whether it be a vacuum of air or one of any other gas ; 

 because, to repeat what I have already stated, unless far 

 more drastic measures are taken — and no mere mechanical 

 pumping will suffice — it is a matter of practical impossibility 

 to remove the insoaked condensed air-films from the surfaces 

 of such highly oxidizable metals as zinc and aluminium. 

 And yet the most illustrious upholder of the contact theory 

 asks concerning Dr. Bottomley's high-vacuum experiments 

 (referred to above, § 1), made in a vacuum of 5^3 mm. and 

 without any application of heat, what would be the efficacy 

 of the supposed oxygen bath in such a case ! * 



§ 13. In face of these results, a perfectly decisive experi- 

 ment seemed almost hopeless to expect. However, I finally 

 decided upon a method of removing the whole of the oxygen 

 which really did offer some hopes of success, and that was to 

 chemically burn away the oxygen. Such a process naturally 

 could not be carried out with aluminium as the positive metal, 

 simply because aluminium has a greater affinity for oxygen 

 than has hydrogen, and it is impossible under any known 

 circumstances to reduce aluminium oxide by means of 

 hydrogen. Iron, however, was a metal that answered all 

 the necessary requirements. The P.D. between it and 

 platinum was sufficiently high (it varies from about 0*3 to 0'5 

 of a volt, according to surface conditions) for the purpose ; 

 the metal could be very strongly heated without fear of 

 fusion or volatilization, and any oxide of iron present or 

 formed in the process of heating could be easily reduced by 



* Phil. Mag. vol. xlvi. p. 82 (1898). 



