The phenomena of electricity depend on a very subtile fluid which is a powerful chymical agent, capable of producing immediate decompositions and new combinations. Galvanism appears to be essentially the same as electricity, differing, however, in some degree in its effects and the mode of its production. Both are to be regarded as repulsive powers.

a. Electricity may be communicated to all substances: by some it is transmitted without any perceivable obstruction, but by others with much difficulty : thence bodies, in their relation to electricity, are distinguished into two classes, conductors and non-conductors: and as it can be accumulated in the latter by friction and other means, these are also denominated electrics; while the former are named non-electrics, to indicate their incapability of being excited.

Metals, plumbago, charcoal, and most liquids, are conductors : all other substances are non-conductors; although many of these, when made very hot, become conductors. Dry atmospherical air is a non-conductor, but when loaded with moisture it becomes a conductor. All electrics are nonconductors, when rubbed, as, for instance, a glass rod, or a stick of sealing wax, with a piece of woollen cloth, attract light substances; and, when a conductor is approached to them, exhibit an appearance of light, attended with a peculiar sound and smell. Some electrics can be excited by simple heating or cooling. It is necessary, however, for obtaining any considerable excitation, that the rubber have some communication with the earth; from which it appears, that the great source of electricity is in the earth; and that excitation consists in the mere transferring of the electrical fluid from one substance to another. By rubbing electrics on one another, the distribution of the electric fluid they contain is altered; and, on separating them, more than the natural quantity remains with the one* and less with the other. The one is then said to be electrified plus, and the other minus, or positively and negatively.

When two bodies are both electrified positively, or both negatively, they repel one another; but if one of them be electrified positively, and the other negatively, they attract one another. Instead of this distribution of the same fluid, the existence of two fluids has been assumed, each of which repels its own particles, but attracts those of the other: and this assumption is more favourable for the explanation of the chymical agency of this fluid.

1 Journal de Physique, xxxiii. 297. 2 Nicholson's Journ. viii. 216.

The chymical effects of electricity seem to depend, chiefly, on its power of producing a sudden high temperature; and this appears to be proportioned to the resistance opposed to its transmission. It often favours chymical combinations; as, for instance, that of oxygen with the metals, and promotes the instantaneous chymical union of gaseous bodies. It also effects chymical decompositions; as, for example, those of water, ammonia, alcohol, and metallic oxides. But for none of these purposes is it employed as a pharmaceutical agent.

b. Galvanism may be regarded as a modification of electricity, in which the fluid is evolved during certain chymical actions. It is transmitted through those substances which are conductors of common electricity, and with the same degrees of facility and rapidity. The metals, charcoal to a certain extent, plumbago, water, the mineral acids, and saline solutions, are perfect conductors; alcohol, ether, sulphur, oils, resins, and metallic oxides, are imperfect conductors; but glass, dried and baked woods, the dry animal cuticle, and dry gases, are non-conductors of the galvanic fluid.

Galvanism is generally excited by arranging two different metals, as, for instance, copper and zinc, and a fluid, as sulphuric acid diluted with twenty or thirty parts of water, in such a manner that the metals touch each other in one part, and have the fluid interposed between them in another.1 Now in this case, as in all others where chymical action occurs between a solid and a fluid, the electrical equilibrium is disturbed, and the latent electricity of the bodies is changed into active electricity. The surface of contact between the acid and the zinc is that at which the chymical action and the developement of electricity take place, whilst the copper acts the part merely of a conductor between the zinc and the fluid on the other side; and, in this manner, a galvanic circle is established. If we suppose z, a, c, to represent zinc, acid, and copper, and the surface of contact between z and a to be that at which the electricity is developed, and c the conductor, the current will circulate in the direction of the arrows from a to c, and thence to z.

In the galvanic trough, the best instrument for developing this modification of electricity, the metals, soldered together 1n pairs, are placed transversely in the grooves of a well-seasoned wooden trough, or one of earthenware, and fixed in with a cement of resin and wax, to prevent any liquid from passing through; after which the diluted acid is poured between the pairs, so that it touches the zinc of one pair, and the copper of another, alternately; the copper side of every double plate looking towards one extremity of the trough throughout the arrangement, and the zinc side to the other extremity. This apparatus is named the Galvanic battery; and the distances between the pairs of soldered plates should be from one fourth to three fourths of an inch each, according to the width, of the trough. The intensity of action of this apparatus, as far as the production of heat is concerned, seems to depend on the size of the plates, or extent of their surfaces, but, for producing chymical decomposition, on the number of plates.

The more improved apparatus now used is a trough of earthenware, a, divided in its length by numerous partitions of the same material.

1 The pile of Volta, which is not now employed, consists of plates of zinc and silver, and pieces of moistened woollen cloth, piled in the order of zinc, silver, cloth; zinc, silver, cloth, for twenty or more repetitions.

Into each of the cells thus formed, and filled with the diluted acid, a plate of zinc and a plate of copper, b, e, are placed near one another, but not so as to touch, and a communication is made, by a metallic arc, between the zinc in one cell and the copper in the next. The whole of these plates are attached to a wooden bar, so that they can be removed at the pleasure of the operator, and supported on hooks fixed in the cross bars of the props e, f, attached to the side of the trough.

As a chymical agent, Galvanism is the most powerful of all the repulsive forces, and is capable of producing decompositions which could not otherwise be effected. By its means, the chymical constitution of the alkalies and the earths has been established, and their bases discovered to be substances before unknown, which have been added to the list of metals.

By placing any body, water, for instance, composed of oxygen and an inflammable substance, in connection with the metallic wires proceeding from each end of a galvanic battery, the oxygen is attracted by the wire which is in the positive state, and repelled by that which is in the negative; while, at the same time, the inflammable is attracted by the negative wire, and repelled by the positive. Hence the components are separated, and obtained in a distinct state. In the decompositions thus effected, substances can be conveyed to a distance, and even through interposed ponderable matter, by the galvanic influence; a result which, however singular, is well ascertained.

Galvanism, like electricity, acts as a stimulus to the living system. Its effects on the animal body are, a sensation of light to the eye; a sensation of acidity on the tongue, and of pain in the muscles; and the excitement of strong muscular action.

It would be entering too much into hypothetical discussion to attempt an explanation of the phenomena of Galvanism. It has not yet been employed as a pharmaceutical agent, except for detecting the presence of bichloride of mercury, employed as a poison. (See Part III.)

On the forces of attraction and of repulsion every chymical, and consequently every pharmaceutical, effect more or less depends A knowledge, therefore, of the laws which regulate these powers is of the greatest importance, and forms the basis of all chymical science, and consequently of all pharmaceutical reasoning.