86 Mr. I. B. Cooke on the Measurement of Chemical Affinity, 



different substances in action ; but, as far as the writer is aware, 

 no attempt has yet been made to analyse these intensities, and 

 to attribute to any two substances the exact amount of force 

 which their act of combination contributes to the general result. 



The inconstancy of the current developed in the simple voltaic 

 cell, — the reduction of its strength arising from the continuance 

 of its own action, — if it do not furnish an argument against the 

 chemical theory, at least seems unfavourable to the attempt to 

 fix any specific constant force, as due to the mutual affinity of 

 any two elements of the series. And the apparent complexity 

 of the more constant forms of electromotors, — the number of ele- 

 mentary substances concerned in their circuits, — appears to forbid 

 the hope of analysing the mere balance of such a multitude of 

 forces, so as to render evident their individual values. 



Let us, however, investigate the sources of the inconstancy of 

 the zinc and copper pair, both immersed in the same fluid. For 

 this pui-pose we shall require to measure, first, the initial intensity 

 of the current of the cell before it becomes in the least degree 

 weakened by its own action; and again, to measure the re- 

 mainder of constant force which may be left after all the sources 

 of inconstancy have expended their action, and before any in- 

 fcrease can be regained by an instant of repose. 



The elegant method of compensation devised by M. Poggen- 

 dorff for the measurement of initial intensities, seems only cal- 

 culated for the most skilful manipulators. To make a perfect 

 and conclusive experiment by this method, requires beforehand 

 a knowledge of the fact which the experiment is intended to 

 determine. And though the exact compensation may be arrived 

 at after a few preliminary trials, the time consumed in these 

 trials, if many cases have to be examined, is inconveniently great ; 

 since if the elements have been subjected to even a slight action, 

 they are no longer admissible for a second experiment, and the 

 whole arrangement must be prepared anew. 



Intensities may also be measured by the use of Professor 

 Wlieatstone's rheostat, and by Ohm's "limit multiplier;" but 

 these instruments act only by virtue of a continuous current, 

 and cannot therefore be trusted to determine either initial or 

 variable intensities. Galvanometers, however, may be constructed 

 sufficiently sensitive to indicate cun-ents incomparably smaller 

 than those for which they are usually employed; and as it 

 seemed probable that the causes of inconstancy in the forces 

 would diminish more rapidly than the forces themselves, so that 

 the former would be wholly imperceptible to an instrument which 

 might yet be sufficiently sensitive to reveal correctly the relative 

 magnitudes of the latter, an attempt was made in this direction. 



So excellent is the conducting quality of metals, that a suffi- 



