is supposed to diverge with less velocity than in air; we shall briefly trace the consequence of this hypothesis.

Let the surface A B be refracting. The wave from the ray LI which enters will, instead of reaching L in a given time, only reach r; the wave from L'I will only reach r': and similarly for the rest. Thus the front of the wave, instead of being at a given instant represented by D l'l, will be represented by Dr'r; and drawing the perpendicular upon it, it will be moving in the direction IE instead of in the direction 17; that is, the direction of the refracted ray lies nearer the normal than the direction of the incident ray. And the simplest geometrical construction would shew that these directions are related to each other according to the ordinary law of refraction (Art. 207).

Such then are the consequences of the incidence of a wave of light on a surface opposing its motion; whether the surface be reflecting or refracting, the ordinary laws of the resulting phenomena may be deduced in a very simple manner from the theory of undulations. It will be impossible here to dwell on the explanation which may be furnished of the phenomenon of refraction being always accompanied by reflexion; of the phenomena of double refraction as the consequence of the different velocities with which the waves diverge in different directions, owing to the different elasticities of the medium; of the phenomena of polarization, considered as the resolution of vibrations, and of the production of fringes, shadows, and spectra; phenomena which, to use the words of Herschel, are so singular and various, that to one who has only studied the common branches of physical optics it is like entering into a new world, so splendid as to render it one of the most delightful branches of experimental inquiry, so fertile in the views it lays open of the constitution of natural bodies, and the minuter mechanism of the universe, as to place it in the very first rank of physico-mathematical sciences, which it maintains by the rigorous application of geometrical reasoning its nature admits and requires.

In conclusion, a few words must be added on an objection which was supposed to be conclusive against the undulatory theory of light. It was objected, that if light be an undulation, as sound is, then ought light to spread in the same manner as sound. That is, the light entering from a luminous point through a small hole ought to illumine the whole room nearly equally, and not illumine one single spot nearly opposite to it; that it ought, like sound coming through the same hole, to spread in every direction. Now it may be assumed as an experimental fact that the lengths of the waves of light are much less than the aperture (Art. 252); the mathematical investigation for the intensity of a disturbance at any point on the theory of undulations shews, that whenever the waves are much smaller in length than the diameter of the aperture the disturbance must be insensible, excepting in front of the aperture; the lengths of the waves of light are much smaller, and the light ought consequently to be, and is, nearly insensible, excepting in front of the hole. This capital objection to the undulatory theory of light may, consequently, be considered as entirely set aside.

CHAP. XI.

ON ELECTRICITY, GALVANISM, AND MAGNETISM.

SECTION I.

ON

ELECTRICITY-TWO SPECIES OF ELECTRICITY ELECTROSCOPES →→ ELECTRICITY BY INFLUENCE-DISTRIBUTION OF ELECTRICITY-LEYDEN JAR-ELECTRIC LIGHT.

256. THE branch of physics on which we are now about to enter occupies a most conspicuous place in modern science. Its existence as a science was scarce suspected before the beginning of the last century; but since that period it has been prosecuted with unabated ardour, and promises to unfold to us many of the most obscure and hitherto inexplicable operations of nature. Thales* appears to have studied some of the phenomena of magnetism, but Gilbert having observed the peculiar phenomena presented by amber, the Greek name for this substance gave rise most naturally to the term electricity.

In examining the properties of bodies, there are some apparently so inseparable from them, or essential to their existence, that we cannot conceive their existence independently of these properties. Thus, all matter is supposed to have impenetrability, extension, weight. There are, however, other properties, which do not at first sight appear † A. D. 1600.

* B. C. 600.

to be so essential to the existence of bodies: thus we might conceive bodies without heat, for this appears to have reference to the particular state in which a body exists, as solid, liquid, or gaseous; and similarly we shall find that electricity is of the same character. It is generally laid down, that no bodies are absolutely devoid of heat, since all appear to be capable of parting with some; it is equally true, that no bodies are devoid of electricity, since all may be made to exhibit signs of it; they may be put into a state capable of exhibiting the phenomena.

257. Phenomena.-The characteristic phenomena of the science of electricity are the mutual attractions and repulsions which some bodies exhibit under peculiar circumstances. We know perfectly well that substances in their natural state have no peculiar property of attracting or of repelling small fragments of gold leaf, tinsel, saw-dust, elder pith, feathers, or any other light substance, whatever its nature; but if a tube of glass, as a vial or a wine-glass, a roll of sulphur, a piece of sealing-wax, amber, or any resinous substance, be rubbed with woollen cloth or silk, it immediately acquires these properties; all light substances are attracted by it, and this attraction is so very powerful, that a light substance will rise many inches, or even a foot, to attach itself to the surface of the attracting body. To the cause of this phenomenon the term electricity is assigned, since, as we have already stated, amber was the substance in which the phenomena were first observed. These attractions are not destroyed or interfered with by the interposition of any body; and since their intensity depends on the extent of the surface, and the force with which it is rubbed, various contrivances, called electrical machines, the construction of which is well known, are employed for the production of the phenomena in a striking and convenient manner. Some of the light substances adhere to the substance which has been rubbed, or the excited body, as it is termed; others are repelled immediately after contact. The rapidity with which bodies rush from a distance to the

electrified body depends on the degree to which it has been excited; and the fact that some adhere to it, others on coming in contact with it are immediately repelled, is one of the phenomena which particularly deserves notice; of this any one may satisfy himself by a stick of sealing wax, and a few strips of the feather of a pen. If the excited body be brought near the face, a sensation is experienced similar to that produced by the contact of a cobweb; when sufficiently excited, and touched by the knuckle or a metallic ball, there is a slight crack, and a spark passes into the body presented to it. When the experiment is performed in the dark, the spark becomes vivid, and is accompanied with a bluish light. Every one is acquainted with the phenomena which present themselves when the back of a cat is rubbed in the dark; these are electrical phenomena, and to the unknown cause is assigned the term electricity, just as the term caloric was assigned to the unknown cause of heat.

258. Electroscopes.-The attraction and the repulsion of a light substance are the characteristics whereby a body is known to be electrified, or excited by friction; and to ascertain this with certainty various instruments, termed electroscopes, have been devised.

Of these the most simple is the electric pendulum, which consists of a small ball of

elder pith, suspended by a silk, or a very delicate metallic thread. When we wish to discover whether a body is electrically excited, we bring it near the ball; if the ball-is neither attracted nor repelled, the body is either not excited at all, or in an exceedingly small degree. The electric needle may also be used for the same purpose; it consists of a most delicate copper needle, poised on its centre, and having a very light pith ball at each extremity. An excited body attracting one ball will make the needle move about