490 report— 1884. 



proceeds to found a number of statements which are true, 1 though 

 scarcely simple ; in fact, they perhaps rather tend to complicate what may 

 be held to be a simple matter. 



Schonbein, in a letter to Faraday published in the ' Philosophical 

 Magazine ' for 1838, 2 throws out a remarkable suggestion with regard to 

 ' ' chemical tendency ' as the possible source of a current, or rather of 

 1 force electromotive.' His language and ideas are in many respects old- 

 fashioned and erroneous ; he uses such phrases as ' a current of tendency,' 

 he supposes currents with no electrolytic power to exist, and of course is 

 not troubled about energy considerations. But I feel little doubt that 

 had he lived later he would have held that, while currents were due to 

 chemical action, electromotive force was due to ' chemical tendency ' ; 

 and this is pretty exactly rny own view of the matter. 



I have only just discovered this Schonbein letter, and I have also 

 found some paragraphs in Faraday which more in detail, and with fair 

 distinctness, express what I believe to be the true view. (See §§ 893-900, 

 'Exp. Ees.' vol. i.) 3 



1 Except, indeed, a doubtful statement at the end of Number 2, and an erroneous 

 bit of reasoning at the end of Number 4, though the conclusion drawn is correct. 



2 Schonbein: Phil. Mag. vol. xii. pp. 225 and 311. The two most striking sen- 

 tences are here extracted : — 



' Before closing my letter, allow me to communicate to you in a general manner 

 the view which I have taken of the subject in question. In the first place, I must 

 tell you that I am by no means inclined to consider mere contact in any case as the 

 cause of the excitement of even the most feeble current. I maintain, on the con- 

 trary, in accordance with the principles of the chemical theory, that any current 

 produced in a hydro-electric voltaic circle is always due to some chemical action, 

 as t«i the idea which I attach to the term "chemical action," I go further than you 

 and M. de la Rive seem to zo ; for I maintain that any tendency of two different 

 substances to unite chemically with one another must be considered as a chemical 

 action, be that tendency followed up by the actual combination of those substances 

 or be it not, and that such a tendency is capable of putting electricity into circulation.' 

 And on page 314 he explains this last phrase, which he has elsewhere called a 

 current of tendency, thus : — 



' As what I term a current of tendency is no doubt in some cases nothing but that 

 electrical state which the voltaists consider to be the effect of their " force electro- 

 motive," or of contact, it appears to me that, from some of the facts above stated, a 

 specific and most important conclusion regarding the theory of the pile can be drawn. 

 Even if we grant to the voltaists our current of tendency to be the effect of mere 

 contact, the facts alluded to prove that such a current does not possess a sensible 

 degree of electrolysing power, consequently that the chemical effects of the common 

 voltaic arrangements have nothing to do with current electricity excited by contact.' 

 * Extract from Faraday's Experimental Researches, vol. i. : — 

 ' (893.) The use of metallic contact in a single pair of plates, and the cause of its 

 great superiority above contact made by other kinds of matter, become now very 

 evident. When an amalgamated zinc plate is dipped into dilute sulphuric acid, the 

 force of chemical affinity exerted between the metal and the fluid is not sufficiently 

 powerful to cause sensible action at the surfaces of contact, and occasion the decom- 

 position of water by the. oxidation of the metal, although it is sufficient to produce 

 *uch a condition of the electricity (or the power upon which chemical affinity 

 depends) as would produce a current if there were a path open for it ; and that 

 contact would complete the conditions necessary, under the circumstances, for the 

 decomposition of water. 



' (894.) Now the presence of a piece of platina touching both the zinc and the fluid 

 to be decomposed opens the path required for the electricity. Its direct communi- 

 cation with the zinc is effectual, far beyond any communication made between it and 

 that metal (i.e. between the platina and zinc) by means of decomposable conducting 

 bodies, or, in other words, electrolytes, as in the experiment already described [that of 

 decomposing iodide of potassium without metallic contact by interposing it on 

 blotting paper between the platinum and the zinc of a simple voltaic cell], 



