rection of Sir Humphry Davy, and been described in his lectures, as chemical professor to the establishment, has acquired a very distinguished reputation. To this have succeeded several other scientific institutions in this metropolis, which have, in different de. grees, contributed towards the same object; the Geological Soci. ety, and the Wernerian Society of Edinburgh; both which, more especially, have been, labouring for some years, in the immediate department to which the observations before us are peculiarly di. rected. From these, and similar establishments, and more particularly from the successful labours of Sir Humphry Davy, we have obtained a more comprehensive insight into the principles of bodies; have assured ourselves, that many of the earths are only metallic oxyds, which may be reduced to a reguline or pure metallic state, by detaching the oxygen, which alone gives them their oxyd form; and have hence been led to believe, that all the other earths, which have not yet been analyzed with the same success, are formed of similar principles. We have been able to decompose the fixed alka. lies; have made no small progress in decomposing ammonia, and the simple combustibles; and have ascertained the very singular fact, that the first of these, whether potash or soda, are themselves metallic oxyds, capable of being reduced, by an abstraction of their oxygen, to metals of an extraordinary character, their levity ena. bling them to float not only upon water or alkohol, but in one in. stance upon naphtha, the lightest fluid we know of. We have also, from the same sources, discovered that oxygen is by no means the only simple supporter of combustion; that there are at least two other substances, chlorine and iodine, (and we have reason to believe there are more) which make a near approach to it in this and various other respects: and which, at the same time have a peculiarity of character that seems to establish them as distinct bodies.

[Editor.

CHAP. II.

ON ELECTRICITY.

SECTION I.

Introduction.

THE study of this interesting and amusing science belongs equally to the chemist, the mechanical philosopher, and the physiologist; for the effects of the electrical fluid are in some instances chemical, in some mechanical, and in some, and peculiarly those which belong to voltaic electricity, physiological. We shall here give it a place in the first of these divisions of science: and shall endeavour to trace the nature of the fluid as it appears when quiescent, or in a state of rest or equilibrium; and when in activity, or in a state of motion. We shall also notice the more curious of the different modes by which it may be accumulated and discharged, and parti. cularly that of the galvanic or voltaic circle.

[Editor.

SECTION 11.

Electricity in Equilibrium.

THE phænomena of electricity are as amusing and popular in their external form, as they are intricate and abstruse in their inti. mate nature. In examining these phenomena, a philosophical observer will not be content with such exhibitions as dazzle the eye for a moment, without leaving any impression that can be in. structive to the mind, but he will be anxious to trace the connection of the facts with their general causes, and to compare them with the theories which have been proposed concerning them: and although the doctrine of electricity is in many respects yet in its infancy, we shall find that some hypotheses may be assumed, which are capable of explaining the principal circumstances in a simple and satisfactory manner, and which are extremely useful in connecting a multitude of detached facts into an intelligible system. These hypotheses, founded on the discoveries of Franklin, have

been gradually formed into a theory, by the investigations of Aepinus and Mr. Cavendish, combined with the experiments and inferences of Lord Stanhope, Coulomb, and Robinson.

We shall first consider the fundamental hypotheses on which this system depends; and secondly, the conditions of equilibrium of the substances concerned in it; determining the mode of distribution of the electric fluid, and the forces or pressures derived from its action when at rest; all which will be found to be deduc. ed from the theory, precisely as they are experimentally observable. The motions of the electric fluid will next be noticed, as far as we can form any general conclusions respecting them; and the manner in which the equilibrium of electricity is disturbed, or the excitation of electricity, will also be considered; and, in the last place, it will be necessary to take a view of the mechanism, or the practical part of electricity, and to examine the natural and artificial apparatus concerned in electrical phænomena, as well as in those effects which have been denominated galvanic.

It is supposed that a peculiar ethereal fluid pervades the pores, if not the actual substance, of the earth, and all other material bodies; passing through them with more or less facility, according to their different powers of conducting it: that the particles of this fluid repel each other, and are attracted by the particles of common matter: that the particles of common matter also repel each other: and that these attractions and repulsions are equal among themselves, and vary inversely as the squares of the distances of the particles.

The effects of this fluid are distinguished from those of all other substances, by an attractive or repulsive quality, which it appears to communicate to different bodies, and which differs in general from other attractions and repulsions, by its immediate diminution or cessation, when the bodies, acting on each other, come into contact, or when they are touched by other bodies. The name electricity is derived from electrum, amber; for it was long ago observed that amber, when rubbed, continues for some time to attract small bodies; but at present electricity is usually excited by other means. In general a hody is said to be electrified, when it contains, either as a whole, or in any of its parts, more or less of the electric fluid than is natural to it; and it is supposed that what is called positive electricity depends on a redundancy, and negative electricity on a deficiency, of the fluid.

These repulsions and attractions are supposed to act, not only between two particles which are either perfectly or very nearly in contact with each other, but also between all other particles, at all distances, whatever obstacles may be interposed between them. Thus, if two electrified balls repel each other, the effect is not impeded by the interposition of a plate of glass: and if any other substance interposed appears to interfere with their mutual action, it is in consequence of its own electrical affections. In these respects, as well as in the law of their variation, the electrical forces differ from the common repulsion which operates between the particles of elastic fluids, and resemble more nearly that of gravitation. Their intensity, when separately considered, is much greater than that of gravitation, and they might be supposed to be materially concerned in the great phænomena of the universe; but in the common neutral state of all bodies, the electrical fluid, which is every where present, is so distributed, that the various forces hold each other exactly in equilibrium, and the separate results are destroyed; unless we choose to consider gravitation itself as arising from a comparatively slight inequality between the electrical attractions and repulsions.

The attraction of the electric fluid to common matter is shewn by its communication, from one body to another, which is less copiously supplied with it, as well as by many other phænomena; and this attraction of the fluid of the first body, to the matter of the second, is precisely equal to its repulsion for the quantity of the fluid, which naturally belongs to the second, so as to saturate the matter. For the excess or deficiency of the fluid in the first body, does not immediately produce either attraction or repulsion, so long as the natural distribution of the fluid in the second body remains unaltered.

Since also two neutral bodies, the matter which they contain being saturated by the electric fluid, exhibit no attraction for each other, the matter in the first must be repelled by the matter in the second; for its attraction for the fluid of the second would other. wise remain uncompensated. We are, however, scarcely justi. fied in classing this mutual repulsion among the fundamental pro. perties of matter; for useful as these laws are in explaining electrical appearances, they seem to deviate too far from the magnificent simplicity of nature's works, to be admitted as primary consequences of the constitution of matter: they may, however, be considered as modifications of some other more general laws, which are yet wholly unknown to us.

When the equilibrium of these forces is destroyed, the electric fluid is put in motion; those bodies, which allow the fluid a free passage, are called perfect conductors; but those which impede its motion, more or less, are nonconductors, or imperfect conductors. For example, while the electric fluid is received into the metallic cylinder of an electrical machine, its accumulation may be pre. vented by the application of the hand to the cylinder which receives it, and it will pass off through the person of the operator to the ground; hence the human body is called a conductor. But when the metallic cylinder, or conductor, of the machine is surrounded only by dry air, and supported by glass, the electric fluid is retained, and its density increased, until it becomes capable of procuring itself a passage, some inches in length, through the air, which is a very imperfect conductor. If a person, connected with the conductor, be placed on a stool with glass legs, the elec. tricity will no longer pass through him to the earth, but may be so accumulated, as to make its way to any neighbouring substance, which is capable of receiving it, exhibiting a luminous appearance, called a spark; and a person or a substance, so placed as to be in contact with nonconductors only, is said to be insulated. When electricity is subtracted from the substance thus insulated, it is said to be negatively electrified, but the sensible effects are nearly the same, except that in some cases the form of the sparks is a little different.

Perfect conductors, when electrified, are in general either over. charged or undercharged with electricity, in their most distant parts, at the same time; but nonconductors, although they have an equal attraction for the electric fluid, are often differently affected in different parts of their substance, even when those parts are similarly situated in every respect, except that some of them have had their electricity increased or diminished by a foreign This property of nonconductors may be illustrated by means of a cake of resin, or a plate of glass, to which a local electricity may be communicated in any part of its surface, by the contact of an electrified body; and the parts thus electrified may afterwards be distinguished from the rest, by the attraction which ty exert on any small particles of dust or powder projected near thean; the manner in which the particles arrange themselves on the