As we have already remai Iced, Dr Wollaston was the first who decidedly pronounced the chemical action of the pile to be the primary cause of its effects ; but in establish ing this point, he did not proceed to explain the nature of the operation, or show what was the train of events which contributed to the final result. This was attempted by Mr Cuthbertson, who, in the essay to which we referred above, after pointing out the inaccuracy of the experiments that were brought forward by Volta in favour of the electric hypothesis, offers some observations in support or the con trary opinion. He conceives that the chemical action of the interposed fluid upon the zinc, alters the electric pro perties of the metal, and disposes it to part with electricity; that this evolved electricity cannot enter into the remainder of the zinc which has not been acted upon, because it re tains its former electric state, but that it is " propelled for wards from the zinc, through the menstruum, to the next adjoining copper in the pile or trough." This effect, how ever, can only happen in a progressive manner, because the fluid is but an imperfect conductor ; and to this he as cribes many of the peculiar phenomena of the apparatus. Dr Henry, in a judicious essay " On the Theories of the Excitement of Galvanic Electricity," (Manchester Mem. ii. 293, 2d Series), observes, that " the explanation of Mr Cuthbertson is unequivocally a valuable supplement to the theory of Volta, inasmuch as it takes into account the effi ciency of chemical menstrua." But, as he farther remarks, it is defective, because it does not explain why " the ac tion of the menstruum is chiefly, if not entirely, exerted in oxidizing and dissolving the zinc plates,•and why the evo lution of hydrogen gas, or of nitrous gas, occurs chiefly at the copper surface." This deficiency was attempted to be supplied by Dr Bostock, who, about the same time, pub lished an essay on the action of the galvanic pile, which he has since considerably extended and modified in such a manner, as to accord with the recent discoveries. (Nish. Journ. iii. 8vo. 9. and 69. Thomson's Annals,iii.52.)—He proceeds upon the principle which was laid down by Dr \Vollaston, that electricity is evolved by the oxidation of metals ; and generalizes it so far as to conclude, that the electric fluid is always liberated when an oxidable sub stance is united to oxygen. In addition to this principle, he proposes to admit the two following postulates, that the electric fluid has a strong attraction for hydrogen, and that when in passing through a chain of conductors, it leaves the oxidable substance to be conveyed through water, it combines with the hydrogen, and is again disengaged from it, whenever it again enters into an oxidable substance. \Ve shall quote the account which Dr Henry gives of this hy pothesis, as it appears to afford a correct, and at the same time a concise view of it.
"To the efficiency of the pile, two circumstances are es sential ; that the electric fluid should be disengaged, and that it be confined and carried forward in one direction, so as to he concentrated at the end of the apparatus. (Plate CCLXIII. Fig. 21.) The first object is fulfilled by the oxidizement of the zinc ; the second, as Dr Bostock sup poses, is effected by the union of the evolved electricity with nascent hydrogen, and by the attraction of the next copperplate for electricity. At the surface of this plate, the hydrogen and are supposed to separate ; the hydrogen to be disengaged in the state of gas, and the electricity to be conveyed onwards to the next zinc plate. Here, being in some degree accumulated, it is extricated in larger quantity, and in a more concentrated form, than before. By a repetition of the same rain of operations,
the electric fluid continues to accumulate in each succes sive pair ; until, by a sufficient extension of the arrange ment, it may be made to exist at the zinc end of the pile, in any assignable degree of force." For a farther account of this sit pothesis, we must refer our readers to the origi nal essay, and more especially to that part of it where the author explains the action of the interrupted circuit in the decomposition of water, and the evolution of the gases at the extremities of the two wires: (Thomson's Ann. iii. 88.) It must be admitted that it satisfactorily explains the phe nomena, and that it accords with all the facts that have hi therto been discovered, but it labours under the great ob jection of being founded upon a gratuitous supposition, of which there is no proof, except the facility with which it explains the appearances.
We think that part of the difficulty which has occurred in forming a theory of the pile, has arisen from our not clearly discriminating between its effects in exciting com mon electricity, and that modification of it which is called galvanism. \Ve have endeavoured to point out in what respect these two actions differ from each other ; and, im perfect as our knowledge is concerning the cause, we con ceive that there is an obvious difference in the effect. Now, it appears to us, that the pile, as it is usually con structed, is both an electrical and a galvanic instrument ; and that when vie attempt to form a theory of its action, we have two distinct sets of phenomena to explain. The power of producing muscular contraction is an electrical effect, that of decomposing chemical bodies a galvanic ef fect ; while that of burning metallic leaves, or igniting wires, probably partakes of both these actions. That the electric and galvanic effects of the pile bear no proportion to each other, that one may exist in a great degree while the other is scarcely apparent, is rendered evident from the experiments of Mr Singer. In examining the power of different kinds of fluids interposed between the plates, he observed, that although some of the effects were rendered more powerful by employing a solution of salt, yet the electrometer was not more affected than with simple wa ter. He even asserts, that in many trials on a very exten :,ive scale, for example, with 1000 pairs of metals, he has " found the electrical effects greatest when the chemical effects have been least. He relates other facts of a simi lar kind, which appear to place this matter beyond all doubt, and to establish a decisive difference between these two operations of the instrument. See Singer's Elem. p. 330.
M. De Luc's experiments confirm and illustrate this view of the subject ; for they not only show this want of proportion between the two effects, but they enable us to separate them from each other. In his second dissection of the pile, we have a powerful electrical instrument, but one which does not produce galvanic effects; and the same may be said of his electric column, which exhibits none of the phenomena that we exclusively refer to galvanism. On the contrary, some of those combinations which have been made by Mr Children, and other experimentalists, where a few large plates were employed, and where a diluted acid was interposed between them, may be considered as pre cisely the reverse of De Luc's column. Here very slight marks of common electricity were manifested, while the most powerful galvanic effects were produced.