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



the amount of gas (in this case hydrogen) in such close contact 

 was extremely small. As the metals cooled — and, indeed, long- 

 after they were cold — they were gradually soaking in and ab- 

 sorbing; hydrogen ; and all the time this was going on the P.D. 

 was gradually becoming more and more negative (see p. 86), 

 until at last the final value, about —0*60 volt, was reached, 

 and this represents the true value of the Volt a effect between 

 iron and platinum in an atmosphere of hydrogen. When, 

 then, 1 pumped out the hydrogen and let in air, the P.D. did 

 not immediately return to its original positive value, although 

 it slowly went back in the positive direction (no more than 

 measuring the contact P.D. of air-covered plates of iron and 

 platinum in hydrogen gives a P.D. of — 0*60 volt) simply 

 because the greater part of the gas in the closest contact with 

 the metals was still hydrogen and not air. The metals wonld 

 probably have to be left for months, and perhaps years, before 

 the air would entirely replace the surface-layers of hydrogen. 

 In fact, it would possibly be as difficult to replace the 

 hydrogen film by an air film as it was to displace the 

 original air film and replace it by hydrogen. Nevertheless, 

 the all-important fact remains that a complete change of 

 atmosphere, unaccompanied, as far as the chemical conditions 

 enable us to judge, by any corrosion or oxidation of the 

 metallic surfaces, induces a complete and fundamental change 

 in the value of the Volta effect, differing not only in mag- 

 nitude but also in sign from the first value. It would be 

 well, if this experiment be repeated, to actually leave the 

 metallic couple untouched for some considerable time (some 

 weeks or even months if necessary) to see whether the 

 maximum positive value of the P.D. would return of its own 

 accord without external aid. In this case I thought I might 

 hasten somewhat the process of replacing the new hydrogen- 

 film again by an air-film by heating the tube. I kept the 

 outside copper sheath red hot for about 15 minutes. When 

 the tube was perfectly cold the P.D. was 



+ 0-22 volt. 



After two hours it remained entirely unaltered at 022 volt. 

 It might be objected that heating in this way oxidized the 

 iron ; but it must be remembered that the plates were still 

 partially surrounded with hydrogen, and before oxidation 

 could take place the hydrogen would have to be driven off. 

 (In any case, at a red heat any oxygen present would unite 

 with the hydrogen rather than with the iron.) This view is 

 supported by the result of a second heating, in this case 

 extending over 20 minutes. Then, after the tube was cold, 



