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III. A Note on Valency, especially as defined by Helmholtz. 

 By Henry E. Arm strong, F.R.S* 



YERY little has been either said or written of late on the 

 subject of Valency — not because the topic is admitted 

 to be exhausted, nor because our views can be regarded as 

 reposing on a fixed basis of fact, but more I believe on 

 account of the feeling being almost universally entertained 

 that little is to be gained by continuing the discussion from 

 our present standpoint. 



My purpose in this note is to call attention to the extreme 

 importance of reopening the discussion on account of the 

 intimate bearing that it has on the work in which the Electro- 

 lysis Committee, jointly appointed by this Section and by 

 Section A, are now engaged ; and to urge that it is time 

 that the gage thrown down by Helmholtz in the Faraday 

 lecture (Chem. Soc. Trans. 1881, p. 277) was uplifted by 

 chemists. 



We are told by Helmholtz that it is a necessary deduction 

 from the fundamental law of electrolysis established by 

 Faraday, that definite, as it were atomic, charges of elec- 

 tricity are associated with the atoms of matter ; that, in fact, 

 a monad bears a single charge, a dyad two, a triad three ; 

 and that when combination occurs, the charges are still 

 retained by the atoms, but neutralize each other — " the atoms 

 cling to their charges, and opposite electric charges cling to 

 each other." I cannot help thinking, however, that Helm- 

 holtz deprives his statement of much of its force and simplicity 

 by adding : " But I do not suppose that other molecular 

 forces are excluded, working directly from atom to atom ;" 

 he is led to do this apparently by being aware of the distinc- 

 tion which it is usual to draw between atomic and molecular 

 compounds. The attempt should at all events be made — and 

 in my paper on " Residual Affinity " I have already ventured 

 the first step — to include both classes of compounds, mole- 

 cular as well as atomic, in the discussion ; indeed it is some- 

 what difficult to reconcile the passage above quoted with the 

 following statement which occurs previously in the lecture : — 

 " The law of the conservation of energy requires that the 

 electromotive force of every cell must correspond exactly 

 with the total amount of chemical forces brought into play, 

 not only the mutual affinities of the ions, but also those minor 



* Communicated by the Author, being a communication to Section F 

 of the British Association at Manchester. 



