188 Prof. Oliver Lodge on the Seat of the 
Various observations regarding the E.M.F. of different cells 
are made in the series of papers still appearing in the Philo- 
sophical Magazine by Dr. Alder Wright and Mr. C. Thompson, 
“On the Determination of Chemical Affinity in terms of 
H.M.F.” 
For much discussion of contact-electricity and for some 
interesting statements of the views of Marianini, Davy, and 
others, refer to ‘ Experimental Researches,’ vol. ii. p. 20 &e. 
From what is there said, it appears that Karsten and Marianini 
held a modified contact theory, placing the E.M.F. at the 
metal-fluid junction; and that Becquerel admitted as a possi- 
bility the efficiency of chemical attraction, as distinct from 
combination, something in the same way as Schonbein. 
Professor Tait in his ‘ Thermodynamics’ lends his powerful 
support to the contact view of the activity of the pile as taught 
by Sir W. Thomson. 
Some work has been done in the direction of observing 
reversible heat-effects at metal-liquid junctions, notably by 
Joule, Thomson, and Bosscha. We shall have occasion to refer 
to this work later. 
Joule in 1841 sent currents through several dilute acid 
voltameters with different electrodes, and measured the excess 
or reversible heat H— RC? generated in the whole cell ; with 
the result that the excess of heat observed is that due to the 
observed back E.M.F. of the cell, minus that concerned in the 
decomposition of water. A table of his results is given by 
Chrystal, Hincyc. Brit. p. 91. For Maxwell on the same 
subject see ‘ Hlementary Electricity,’ p. 146. 
Thomson (Math. and Phys. Papers, pp. 496, 503) says 
that, of two decomposition-cells, one with zine cathode, the 
other with platinum cathode, the former showed the most 
heat when the same current was sent through both. Separa- 
the circuit between the zine and platina immersed separately into the 
dilute acid; for if in the above experiment the platina wire be retained in 
metallic contact with the zinc plate, and a division of the platina be made 
elsewhere, then the solution of iodide placed there, being in contact with 
platina at both surfaces, exerts no chemical affinities for that metal; or, 
if it does, they are equal on both sides. Its power, therefore, of forming 
a current in opposition to that dependent upon the action of the acid in 
the vessel is removed, and only its resistance to decomposition remains as 
the obstacle to be overcome by the affinities exerted in the dilute sul- 
huric acid. 
(896) This becomes the condition of a single pair of active plates 
where metallic contact is allowed. In such cases, only one set of opposing 
affinities are to be overcome by those which are dominant in the vessel ; 
whereas, when metallic contact is not allowed, two sets of opposing 
affinities must be conquered (894).” 
