30 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



In repeating the experiment two improvements were introduced. For 

 fear that some silver might have volatilized and contaminated the iron, the 

 silver-wire support was replaced by a simple mechanical device, one side of 

 the porcelain plate supporting the iron being held in place by a pipe-stem, 

 while the other side was attached to an iron wire running through a lubri- 

 cated hole in the stopper. By pulling the wire the iron could be dropped. 

 Furthermore, quick and effectual cooling was obtained by surrounding the 

 outside of the base of the porcelain tube, containing the iron cooling-plate, 

 by a mixture of chloroform and solid carbon dioxide. Time was allowed for 

 the thorough cooling of the inclosed iron plate. The temperature of porous 

 iron was thus almost instantaneously lowered from i,ioo to 75 or there- 

 abouts. Great pains was taken to exclude the vapor of water, which would 

 have condensed in the cold part of the tube and vitiated the experiment. 

 This was accomplished by running a stream of pure dry hydrogen through 

 the tube for many hours. 



As soon as possible after the fall of the porous iron, which took place 

 this time in an atmosphere of hydrogen, the pieces were taken out, immersed 

 in ferrous sulphate, and measured in the usual way. In six minutes after 

 immersion the potential was 0.790, in twenty minutes 0.796, and in an hour 

 0.798. After a day it settled down to perfect constancy at 0.793 y k, essen- 

 tially identical with the result of the previous experiment. This is not 

 enough above the usual value (0.787) obtained from iron slowly cooled from 

 the same high temperature to allow the conclusion that the sudden cooling 

 had made any considerable difference in the potential or difference in internal 

 structure. 



It becomes now an interesting question as to which of the many values 

 for the gradually changing potential of the cell Fe, wFeS0 4 , n/ioKCl, HgCl, 

 Hg correspond to the true values of the solution tension of the different 

 forms of iron. Consideration shows that the final high values are probably 

 to be taken in each case, for several reasons. In the first place, these values 

 are approached asymtotically, and are then maintained at a constancy for a 

 long time. For example, one sample of iron which in 16 hours had given 

 in the cell a potential of 0.793 volt, after 150 hours more was still practically 

 unchanged at 0.792 volt. In the next place, the different kinds of iron, 

 although sometimes beginning at impossibly low values, all finally rise to 

 nearly the same point. Again, it is not conceivable that the iron should 

 gradually raise itself to a potential above its true value and maintain itself 

 there in a reversible reaction with a large excess of both iron and ferrous 

 sulphate at hand, while it is easy to see that a coating of oxygen would lower 

 the potential at first. The conclusion is, moreover, reinforced by the fact 

 that the same potential is attained from the opposite direction by the gradual 



