Waving, however, the farther discussion of this point, which indeed can only be decided by experiment, we must recur to the question already stated, respecting the essen tial difference between galvanism and common electricity ; and, conceiving it to be ascertained, that in the production of the former, a chemical action takes place, which is not necessary in the latter, we must next enquire, in what way this chemical change of the substances imparts to the elec tricity that particular state or modification which we style galvanic. With respect to the nature of this chemical change, experimentalists are generally agreed ; as to the metals, it consists in the oxidation of that metal which pos sesses the strongest attraction for oxygen ; and with re spect to the fluid interposed between the metals, it consists in its decomposition, the oxygenous part being attracted to the most oxidable metal, and the alkaline to that which is the least oxidable. Although, as we 'have already had oc casion to remark, there are various galvanic combinations, into which only one metal enters, or even some entirely without metals, yet, as the most powerful and complete cir cle is that which consists of two metals with a fluid inter posed, we shall confine our illustrations to this form of the apparatus.
We may consider it as proved by a number of experi ments, which have been stated in the first part of this arti cle, that the electricity, as it is evolved by the different gal vanic combinations, always exists in what has been styled a state of low intensity ; and that, to whatever extent we in crease the apparatus, and however powerfully it acts, still the intensity is but little augmented. Unfortunately it is still a doubtful point of theory, upon what the intensity of electricity depends, or in what it precisely consists. Some writers have ascribed it to a greater or less concen tration of the fluid ; some to a difference in the velocity of its motion, or in the strength of its affinity for the surround ing bodies ; and others to its containing a greater or less portion of caloric. For the present, we must rest satisfied with admitting the fact of the low intensity, as manifested by the phenomena, without being able to explain its cause ; and we may next proceed to enquire, whether there be any circumstances in the different methods of exciting or pro ducing electricity, by the machine or the pile respectively, which should cause the first to develope the fluid in a high er, and the latter in a lower state of intensity.
And here, it must be confessed, we have little to direct our inquiries but conjecture and uncertain analogy. Of these, however, as being our only guides, we must make the best use that lies in our power. It is generally agreed, that all bodies possess a certain quantity of electricity, which is said to be natural to them, and which, while it remains undisturbed, manifests no indications of its exist ence. There are many processes which alter the state of this natural electricity, by which it is extricated from one body, and may be transferred to others in the neighbour hood. But this additional portion, being more than their natural share, seems to be retained by them with difficulty, and is ready to fly off in all directions, in order to restore the equilibrium. This may be considered as descriptive of what occurs in the operation of the common electrical machine, where, by the friction of the rubber against the cylinder, a portion of the electric fluid is carried cif from one or both of them, and is transferred to the conductor. From
the conductor it may be communicated to a variety of other bodies that are placed within the sphere of its influence ; but, in all these cases, it is retained by them for a certain space of time only, and is continually passing off, more or less rapidly, to all the surrounding bodies.
But besides this temporary transfer from one body to another, without their undergoing any farther alteration, they occasionally experience a more permanent change in their electrical state, when, in consequence of their acquir ing different physical and chemical properties, their ca pacity for electricity is entirely altered. When their ca pacity is diminished, a more gradual, but more continued discharge of the electric fluid takes place ; and in this ap pears to consist the essential action of the pile, as contrasted with that of the machine. In the action of the machine, by which the electric fluid is set at liberty, and transferred from one body to another, no change appears to take place in the substances employed, except the alteration in their respective quantities of electricity. Their attraction for it is neither increased nor diminished ; and, consequently, they have a tendency, the one to lose, and the other to acquire, the electricity which has been thus, as it were, forced into the one, and out of the other. According to the nature of the action by which the electricity is evolved, whether the substances experience any permanent change in their ca pacity, or whether their equilibrium is merely disturbed in a temporary manner, the state of the fluid appears to be affected, so as to cause a difference in its intensity.
When we employ the machine, the electricity that we pro cure appears to be in a highly elastic state, its particles strongly repulsive of each other, and at the same time not disposed to enter into a permanent union with other bodies. The galvanic electricity which we procure from the pile, is more readily united to other bodies, and has a tendency to form new combinations with them, which is so powerful as to counteract some of the strongest chemical affinities. At the same time, it exhibits less of what may be called me chanical action : its particles are less repulsive of each other ; its motions appear less rapid; it causes less commotion in its passage from one body to another ; and although its ultimate effects are more powerful, it seems to act with less violence. The one may be compared to a small quan tity of an agent highly concentrated ; the other to a larger quantity, but in a state of greater dilution. The phenome na of electricity, as excited by the common machine, depend upon the attraction and repulsion of the electric fluid, and its passage from one body to another ; while the most im portant actions of galvanic electricity result from the chemi cal changes that it produces in the composition of bodies. The excitation of common electricity is not necessarily at tended with any permanent alteration in the state of the substances that are employed in producing it. It is usually developed by the mechanical aid of friction, and the same apparatus may continue to be employed for an indefinite length of time. Friction, on the contrary, has no effect in the production of galvanic electricity ; it requires a chemi cal change in some part of the apparatus ; and the indi vidual parts which have been employed in generating it ac quire new properties, and are incapable of any farther gal vanic action.