the Tliermo-Electric Properties of Liquids. 



535 



tilde of the current varies with each different base and acid ; but in the 

 few cases in which electrodes of different metal were employed with the 

 same liquid, as with platinum, copper, and iron in solution of sodic 

 carbonate in ISTos. 118-164 and 165 ; with platinum and iron in solution 

 of sodic hydrate of Nos. 142 and 1 66, and with iron and platinum in 

 solution of potassic cyanide of Nos. 145 and 167, the quantity only, and 

 not the direction, of tbe current was affected. This agrees with the 

 results obtained with the cylinder apparatus, in which a greater variety 

 of metals was employed. The only apparent exception to this state- 

 ment was with iron in solution of potassic cyanide of No. 109 ; but 

 the contradictory result obtained in that case was evidently due to 

 the liquid acting upon the washers. 



Cause of the Currents. 



The heat applied to one of the plates must either act as the real 

 cause of the currents, or merely as an exciting cause, by liberating from 

 a potential state another force which produces them. If it acts as the 

 real cause, the effect will be proportional to the heat applied, except in 

 those cases where some condition (such as conduction resistance, or a 

 reversal of electric polarity) exists to prevent or diminish it. If, how- 

 ever, it operates merely as an exciting cause, the current would either 

 be large in proportion to the amount of heat applied, or vary in a 

 different ratio to the temperature, because the liberated force would 

 probably obey a different and higher law of increase than that of the 

 temperature. The substances also, having evolved a portion of their 

 stored-up power, would be in a different potential condition after an 

 experiment to what they were before it. If, however, we examine all 

 the results, we find that in nearly the whole of the instances the cur- 

 rents increased in magnitude (usually in approximate proportion) with 

 the temperature, and the metals and liquids after an experiment were 

 in precisely the same physical and chemical state as they were before it, 

 and might therefore be used again for the same experiment any number 

 of times without loss of efficiency. 



Heat alone, or chemical affinity excited by heat, appears to be the only 

 probable cause of the currents. That chemical action, started by heat, 

 did not produce them, appears certain — 1st, because no corrosive effect 

 (shown by loss of brightness) was manifested upon the plates, even 

 after many experiments, except in certain cases, and this was quite a 

 delicate test ; * nor in the great bulk of the instances did any other 



* As the plates were burnished, a change of brightness was a much more delicate 

 test than any loss of weight, I therefore did not test for corrosion bj weighing them. 

 To generate a current also of equal amount to that produced in some of the in- 

 stances, an appreciable amount of metal would have been required to dissolve. 

 Further ; no signs of dissolved platinum, gold, or palladium were observed in the 

 residues obtained on evaporating and crystallizing the various liquids, in those cases 

 where no signs of corrosion had been observed upon the plates. 



