66 Messrs, Ay Hon and Perry on Frof. Exner's 



themselves intimately with the consequences of the chemical 

 theory; they contradict entirely the contact theory." For 

 he reasons : — " The contact theory requires that the electro- 

 motive force of a Smee's element should hegin with a value 

 0'73 of that of a DanielPs cell " [this is derived from the 

 heat of combustion of zinc in oxygen, the combination of the 

 oxide \x\\h sulphuric acid, minus the energy required for the 

 decomposition of an equivalent quantity of water] " and then 

 sink to a lower value, which depends on the amount of pola- 

 rization — that is, on the amount of resistance, — and, further, 

 that it depends on the nature of the negative plate. The 

 chemical theory, on the contrary, requires that the Smee's 

 element should first have an electromotive force between 0*732 

 and 2*15 of that of a Daniell " [2*15 is derived from only 

 considering the heat of combustion of zinc with oxygen and 

 the combination of the oxide with sulphuric acid], " and that 

 this value should fall to 0*732 of a Daniell and then remain 

 perfectly constant, no matter what be the metal forming the 

 negative plate, as long as it does not give rise to chemical 

 changes. Further, the value 0*732 of a Daniell must also 

 depend on the resistance of the element." 



Now what is correct in this was all given by Sir Wra. 

 Thomson as far back as 1851; but the greater part is incorrect. 

 First, it is quite a mistake to say that " the contact theory 

 requires that the electromotive force of a Smee's element 

 should begin with a value 0*73 of that of a Daniell's cell." 

 What the " summation law"* of the contact theory says is, 

 that the electromotive force of a complete cell is equal to the 

 algebraical sum of all the differences of potential, each mea- 

 sured separately, at the separate contacts — of metals with any 

 layer of gas on them, gas with liquid, one liquid with another, 

 &c; and what these latter are in a Smee's water element, for 

 example, can be found from the table at the end of our paper t. 

 For, taking water, we have 



Pt | H 2 . . 0«285 to 0-345 volt*, 

 H 2 | Zn . . -0-156 „ 0*105 „ 

 Zn | Pt . . 0*981 „ 0-981 „ 



1*110 to 1-431 volt, 

 Mean . . . 1*270 volt, 



* " Contact Theory of Voltaic Action," Part I., by Professors W. E. 

 Ayrton and John Perry, Proc. Roy. Soc. No. 186, p. 23, 1878. 



t ''Contact Theory of Voltaic Action," Part III., by Professors W. 

 R, Ayrton and John Perry, Phil. 'Trans. Roy. Soc. parti, pp. 15-34, 

 1880.' 



\ The variations observed in measuring- the electromotive force of con- 

 tact of a metal and a liquid are due to the effect of gas absorbed, and being 



