THEORY OF GALVANISM.
• to the plan which was laid down, we must now proceed to give an account of the theories and hypo theses that have been formed to explain the phenomena of galvanism. We have had occasion to allude to many of these in the course of our historical sketch ; and the rea der will, in some degree, have anticipated our opinion re specting them. The subject divides itself into several branches, partly corresponding with the progress of our knowledge of the facts that were gradually developed, and partly depending upon the supposed relation of galvanism to the other departments of natural philosophy.
In this concise view of the science, we shall not think it necessary to enter into the merits of the earlier specula tions, that have been superseded by later discoveries. Of this nature is the original hypothesis of Galvani himself, that the convulsions which he observed in the muscles of frogs were produced by a new and peculiar agent, residing in the body, to which he gave the name of Animal Elec tricity. Although there are some few cases which seem to militate against the supposition, it must, upon the whole, be regarded as being decisively proved, that all the pheno mena which we stile galvanic, depend merely upon the action of electricity, modified by the manner in which it is produced or excited.
Hence arises an interesting question, and one which lies at the very foundation of all our future inquiries: How does galvanism differ from common electricity ? This question may refer both to the nature of the phenomena themselves, and to the Ineans employed for their produc tion. We may define galvanism, either by enumerating the specific characteristics of those events which we class together under this title ; or we may show how they have all a reference to each other, from the similarity of the processes that are employed for their developement. The definition that we have given at the commencement of the article, may be regarded as sufficiently correct and corn prehensive, without exceeding the limits to which a defini tion ought to be restricted. It appears to include every action of bodies upon each other, which is usually consi dered as belonging to this particular branch of natural philosophy ; while it excludes those that are, by common consent, referred to a different department. It is, however, in some cases, difficult to draw the exact line of distinction between electricity and galvanism, and indeed we may doubt, whether any precise distinction actually exists. For, as it is conceived that they both depend upon the same agent, having merely experienced some modifica tion in its nature, or mode of action, we must conclude, that there may be some intermediate or indeterminate state, which might be referred to one or the other with almost equal propriety.
To recur then to the former definition : u Galvanism is a series of electrical phenomena, in which the electricity is developed without the aid of friction, and where we per ceive a chemical action to take place between some of the bodies employed." This definition may perhaps be thought to limit the science too much, and to remove from it many facts, which have always been regarded as galvanic. For example, a great number of the original experiments of Gal vani himself, and his immediate contemporaries, where contractions were excited in the muscles of animals, by the application of the two metals, many of those of Fowler, and the first set of Volta's experiments, would, according to this definition, be reduced to the effects of common elec tricity. To this objection we may reply, that wherever moisture comes in contact with the zinc, or more oxidable metal, it is not improbable that some chemical action is produced, hut that it is very slight, and has therefore not been noticed. If, however, upon a strict examination, it is found not to be the case, and that there is actually no change in the chemical condition of any part of the apparatus, it must be admitted, that, according to our present ideas, the phenomena are not to be referred to galvanism. The lirst unequivocal experiments where the chemical effects were observed, and were connected with the electrical condition of the substances, are those of Fabroni's ; and it was not until Volta's discovery of the pile, that we were put in pos session of a method by which we were enabled to examine, with any degree of accuracy, the relation between these two actions. Even if we find it necessary to conclude that Galvani, although he had the good fortune to have his name associated with a new department of science, did not witness any of the facts to which we now apply the term, the contradiction will be more apparent than real ; and we must not permit the mere circumstance of names to influ ence our opinion respecting the essential nature of things. The present state of our knowledge seems, however, to warrant the conjecture, that the action of the two metals on the parts of animals, is strictly galvanic, i. e. accompanied by a chemical action on the metals and the fluids, so as to reduce it within the limits of the proposed definition.