the cornea, of perfect transparency, placed exactly in the fore part of the globe; then, behind this, is the circular curtain, the iris, with its opening, called the pupil, dilating and contracting, without consciousness of the person, to suit the varying intensity of light; and exactly behind the iris, again, is the crystalline lens, possessed of the remarkable power of bending the entering light, to form on the retina perfect pictures or images of all the objects in front, the most sensible portion of the retina being just where the images fall. Of these parts and conditions, had any one been otherwise than what it is, the whole eye had been useless, and light useless, and therefore the whole world useless to man. Then, again, we observe that there are two of these optical organs which have so entire a sympathy that they act together as one doubly powerful; and, finally, the sense of sight continues perfect from the birth of the individual to maturity, although during growth a continual adjustment to one another of all the delicate parts has to be maintained; and the pure liquid which distends the eyeball, if rendered turbid by any accident, is, by the actions of life, although its source be the thick red blood, gradually restored to the purest transparency. The mind which can suppose or admit that, by chance or without design, during any length of ages, one single such apparatus of vision, as above described, could have been produced, with powers of growth and reparation, must surely want the higher faculties of reason which distinguish man from the lower animals.

SECTION III.-ELECTRICITY.

ANALYSIS OF THE SECTION.

ELECTRICITY, a subtle natural agency or affection of matter, may be excitea (i.) by mechanical force, (ii.) by chemical action, (iii.) by heat, (iv.) by magnetism, or (v.) by the induction or neighbouring action of other similarly electrified matter. All electricity has a remarkable duality, polarity, or double-sidedness of character.

(i.) FRICTIONAL ELECTRICITY, or that excited by mechanical means, manifests itself in mechanical attractions and repulsions of light objects, and when produced in quantity by electrical machines, has powerful luminous, heating, physiological, and other effects. By means of the LEYDEN JAR, it may be accumulated, condensed, or stored to a high degree, so as to resemble the natural atmospheric phenomenon of Lightning in its effects. ii.) GALVANIC, or VOLTAIC ELECTRICITY, or that which is excited by chemical action, differs from the preceding in its intensity, appearing not with sudden discharges, but in a continuous current or flow, which requires special apparatus for its detection. Its origin and its effects are alike molecular; its uses are more extensive, as its tractability is greater, than the former, travelling along metallic wires to any distance, and equally ready to exhibit its effects at any spot in the circuit. Born of chemical action, it has important chemical powers of effecting decomposition of compound substances; the special art of electro-metallurgy has been developed from the electrochemical effects of the galvanic battery.

(iii.) THERMO-ELECTRICITY, or that excited by heat, is similar in character to the former; but the transformation of heat-energy into the electrical form by any apparatus yet devised has not been so complete as that of chemical energy. The chief importance of this subject centres in the Thermopile, a most sensitive detector of heat-changes.

(iv.) MAGNETISM, formerly ascribed to a special force, is now identified with electricity. That the magnetism of our globe, as well as of steel permanent magnets, may be due to electrical currents, is rendered highly probable by the fact that all magnetic phenomena may be imitated by electrical currents suitably disposed, and electro-magnets of any strength may be produced by the galvanic current circulating round soft iron. Magnets and currents have a reciprocal action, which has been reduced by Faraday to very definite laws.

(v.) By INDUCTION, or action at a distance, electricity may be generated from

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Discovery of Electricity.

mechanical force to almost any degree; some electricity or permanent magnetism being required as capital to start with; a large class of machines depending on this principle have been recently devised; and the effects produced have been in some instances marvellous.

Among the manifold applications of Electricity, the Electric Telegraph holds the first place; its recent development by sea and land forming an era in the history of civilization.

Electricity, a subtle natural agency or affection of matter,

may be excited, first, by friction or mechanical force.

928. When the student opens a book on Electricity, and finds in the first page a mystery thrown around this subject by the avowal that the ultimate cause or electrical agent is not yet fully known, he is apt to imagine that there is some special difficulty connected with the comprehension of a subject on which professed electricians do not agree. He may reflect, however, that the leading laws of gravitation, light, and heat were detected and turned to important uses, before any accurate ideas had been formed as to the character of the respective natural agencies.

Though we have not yet discovered the final cause of gravitation, yet the discovery of its laws by the immortal Newton, has remarkably increased men's knowledge of astronomy, and advanced many of the useful arts dependent thereupon. James Watt did not know completely the nature of heat, when he was led to the construction of the steam-engine, which has become almost synonymous with modern mechanical power. So, although men may entertain different views as to the existence of electric fluids, they have learned enough of the laws of this wonderful agent called electricity, to be able to protect their ships and buildings, as well as their lives, from the destructive thunderbolt, and to almost annihilate distance by that wonder of modern times, the electric telegraph.

We shall in the present section deal merely with the leading experimental facts of the science, which is all that the purpose of the present treatise requires.

The earliest recorded observation of an electrical phenomenon we owe to Thales, a Greek philosopher, who lived 2500 years ago. He noticed that amber (in Greek called elektron), when rubbed on woollen cloth, attracted small light bodies; and the name electricity (or amber-action) was subsequently given to the whole class of phenomena of which this fact noticed by Thales was but one particular

Elementary Experiments.

693

instance. The science, being wholly an experimental one, slept in undeveloped embryo till 1600, when Dr. Gilbert, a physician of Queen Elizabeth, recalled attention to it in a meritorious work on magnetism and kindred phenomena.

B

929. Apparatus.—It is interesting to know how few the simple objects are which form an apparatus sufficient to exhibit the furdamental facts or truths of electricity, and that these are at hand in every ordinary dwelling-house. A person aware of this, and not choosing to make the easy experiments, would show singular disregard of important natural knowledge. The requisites are—

I. A silk handkerchief, or a piece of woollen cloth.

2. A piece of glass tube, as B (fig. 244), or a long narrow phial. Some wine

glasses, G, H.

Fig. 244.

3. Small round pieces of cork or balls of elder-pick, as N, harging from any support by threads of white silk.

4. A stick of sealing-wax, or of shell-lac, C.

5. A common fire-poker, or any rod of metal, L.

M

G

Fig. 245.

930. Experiments.—(a.) If a glass tube, B, or the bottom of a wine-glass, well dried and warmed, be rubbed briskly with a silk handkerchief also made dry and warm, and if it be then held towards the cork ball, N (fig. 244), hanging from the support, A, by a thread of white sewing silk, the ball will be drawn or attracted from the position, A N, to that of A g, nearer to the glass, and there it will remain while the position and electrical state of the glass continue. If the glass be moved about, the ball will move to follow it.

(b.) If then the electrified glass and ball are allowed to touch

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Electric Attraction and Repulsion.

each other, the ball receiving part of the electricity would instantly dart away or be repelled by the glass; and if the glass be moved towards it from any side, it will elude the approach, like a living thing terrified.

(c.) After the contact of the glass and ball, the ball will have acquired the power of attracting any other light thing placed near it, as a second ball like itself, and if these be allowed to touch, they will instantly repel mutually, as the glass and ball did in the last experiment.

(d.) A glass tube being a non-conductor of electricity, if held by one end, loses its charge only slowly, and chiefly to the air; but if a hand be passed along its surface, the hand, being a conductor, carries away the electricity, and the natural or normal state of the tube is restored. So an electrified cork ball, which attracts and is attracted by things around, if allowed to touch a finger, loses its electricity and comes to rest in the natural position of the pendulum.

(e.) If, instead of one ball hanging from the support, there are two, and in contact as at the point L, on touching them with the electrified glass, they dart asunder, and if the glass be placed between them, they separate still further.

(f.) If a metallic rod, D, as a common fire-poker or a length of thick wire (both being conductors), has attached to it, at one end, L, two pith-balls hanging by fine wire or linen thread, and if the rod be insulated by being supported on two wine-glasses, G and H, as shown above, and if then the end of the rod, M, be touched by the excited glass tube, the two balls at the end, L, will instantly repel each other as if the glass had touched them directly, showing that the metallic rod gives instant passage to the electricity from the glass to the balls. The rod or wire might be of great length, and still the effect would be produced.

(g.) If the electricity be strongly excited on a large glass tube in the dark, the surface is often slightly luminous, and minute sparks are seen to pass from the glass to knuckles held near, and a slight crackling noise is heard, showing that all our senses can perceive excited electricity

(h.) The set of experiments described above may be performed more feebly by using the silk with which the glass had been rubbed, as the electrifier, instead of the glass itself, but with this singular difference, that the bodies which are attracted by the glass are repelled by the silk, and vice versa. One of the rubbing bodies seems to gain that which the other loses.