ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 505 



air contact forces. Bat we have no right to assume that even Volta 

 effects will agree with calculation particularly well so long as our data 

 are so slender. They have no chance of accurately agreeing unless the 

 metals used are pure and perfectly clean — a most difficult condition to 

 attain for even a few seconds. 



Before leaving this subject it may be well to point out that, whereas 

 the calculation of a Volta effect depends on data obtained hy allowing 

 oxygen atoms to approach the metal completely and actually combine, 

 the experimental determination- hy Kohlrausch's and similar methods 

 depends on letting the atoms approach somewhat nearer to one metal and 

 recede somewhat farther from another ; while the compensation form of the 

 experiment employed by Pellat and others depends upon forcing back 

 and restoring the atoms to their original or standard positions. Now if 

 the views here just expressed have any sense or signification whatever in 

 actual fact, it would be very natural to suppose that the numbers obtained 

 in these three ways might be slightly different. But to specify the direction 

 in which we should expect the differences, if any, to lie would require us 

 to have made up our minds as to the probable variation of chemical 

 attraction with distance. Assuming an inverse variation, the Pellat 

 method should give the least, the Volta or Kohlrausch method the next, 

 and the calculation method the greatest, value for the Volta effect. 



Bat all these ideas complicate the matter somewhat, and they are quite 

 possibly unnecessary. If it be considered that we have no data at present 

 it may be permitted to work on the simplest hypothesis, viz. that the 

 step v is independent of how nearly chemical action has occurred — that it 

 is the same for atoms straining at one another at their normal distance as 

 for atoms on the verge of combination. And it may be argued in favour of 

 this view that we really have some data, viz. these. 



If it were not trae, results obtained by Pellat's method could not be 

 expected to agree exactly with those obtained by Kohlrausch's (of which 

 Ayrton and Perry's or Clifton's may be taken as the best examples). Now 

 results obtained by these different methods do agree very fairly well ; exact 

 agreement cannot be predicated, for the most trifling circumstances cause 

 large variations in the Volta effect, but no decided disagreement is observ- 

 able. Again, if it were not true, the Volta effect observed when two 

 metals far apart in the series (e.g., zinc and platinum) were employed 

 would be inconsistent with the results obtained by using metals nearer 

 together, say zinc and tin, or tin and platinum ; and if this were so, the 

 metals conld not be arranged in the linear series which eighty years ago 

 \ olta showed they could be. These arguments throw no light on what 

 may happen just before actual combination, still they are encouraging so 

 far as they go. 



17. Let us therefore endeavour to suppress further qualms, and calculate 

 a series of metal-air contact forces from the heats of combustion ; remem- 

 bering that all we have to do, in order to convert heats of combustion per 



dyad gramme-eqnivalent into volts, is to divide by X ;thatis. 



42xlU G 

 by 46000. 



But the decision as to what numbers we shall take to represent heats 

 of combustion is a matter of some difficulty, for not only do the numbers 

 obtained by different observers for the same reaction differ, sometimes 

 considerably, but it is not obvious when different oxides are formed 



