becoming more diffused as the air becomes more rarefied, until at length the whole receiver is pervaded by a beautiful bluish light, changing its color with the intensity of the transmitted electricity, and producing an effect which with an air pump of considerable power, is pleasing in the highest degree.*

When a charged jar is placed under the receiver of an air pump, as the exhaustion proceeds, a luminous current flows over the edge of the jar from the positive to the negative side, until the equilibrium is restored.

733. Electric light exhibits a very beautiful appearance, as it passes or flows, through the Torricellian Vacuum. The color is of a very delicate bluish or purple tinge, and the light pervades the entire space. But the most pleasing exhibitions of this kind, are made by forming an artificial atmosphere of vapor in the Torricellian tube. Ether or alcohol, passes into the state of vapor, when the pressure of the atmosphere is removed; and accordingly, on introducing a drop of one of these fluids into the Torricellian vacuum, it immediately evaporates and fills the void. If, now, a strong spark be passed from the prime conductor through this vapor, the spark will exhibit various colors: in ether, it is an emerald green, or mingled red and green; in alcohol it is red or blue; but the colors vary somewhat with the distances at which they are seen.

734. Sir Humphry Davy performed a number of experiments, on the passage of electricity through a vacuum, of which an account is given in Philosophical Transactions for 1822. He succeeded in forming a Torricellian vacuum quite free from air, but in such cases, a small portion of the mercury itself is converted into vapor, and from this he could not free the empty space. In all cases when the mercurial vacuum was perfectly free from air, it was permeable to electricity, and was rendered luminous by either the common spark, or the shock from the Leyden Jar. But the degree of the intensity of this phenomenon depended upon the temperature. When the tube was very hot the electric light appeared in the vapor of a bright green color, and of great density. As the temperature diminished, it lost its viv

* Singer.

idness, and when it was artificially cooled to 20° below zero, it was so faint as to be visible only in the dark. In all cases, where the minutest quantity of rarefied air was introduced into the mercurial vacuum, the electric light, changed from green to sea green, and by increasing the quantity of air, it changed to blue and purple. Also when the temperature was low, the vacuum became a much better conductor.

A more perfect vacuum was formed by means of melted metals, as tin, of a more fixed nature than mercury, and therefore not liable

to impair the vacuum by vapor of their own. A vacuum being made by means of fused tin, the electric light at temperatures below zero, was yellow, and of the palest phosphorescent kind, requiring almost absolute darkness to be perceived; nor was it perceptibly increased by heat. When the temperature was diminished, the electrical light (transmitted through vapor of mercury) diminished also till the temperature was reduced to 20°; but between 200 and -20° it seemed stationary.

Unless the electrical machine was very active, no light was visible during the transmission of electricity; but that the electricity passed was evident, from the luminous appearance of the rarefied air, in other parts of the tube.

From these and various similar experiments related by Davy, it seems demonstrated, that electricity is capable of passing through a perfect vacuum, but that the light emitted depends upon the vapor or air through which it passes, and that if the vacuum were perfect, no light whatever would appear.*

In condensed air, on the contrary, the spark passes with greater difficulty than ordinary. In such case, also, its whiteness, and brilliancy are augmented, and its course is zigzag. These appearances are even exhibited by passing the spark through confined air, of only the ordinary density.

735. The colors of the spark, are pleasingly varied by passing it, in a condensed form, as in the Leyden Jar, through media of different kinds. The experiment is performed by making the given body

* Phil. Trans. 1822, or Thomson, Qutlines, p. 470.

form a part of the circuit of communication, between the inside and outside of the Leyden Jar. A ball of ivory in this situation exhibits a beautiful crimson; an egg, a similar color, but somewhat lighter; a lump of sugar, gives a very white light, which remains for some time after the spark has passed; and fluor spar exhibits an emerald green light, or, in some cases, a purple light, which also continues to glow in the dark for some seconds. The great intensity of the light is shown by the strong illumination which the sparks in the jar communicate to bodies slightly transparent. Thus an egg has its transparency greatly increased; and if the thumb be placed over the space which separates the two conducting wires that communicate with the two sides of the jar respectively, the illumination is so powerful, that the blood vessels and interior organization of the organ may be distinctly seen.

736. Metallic conductors, if of sufficient size, transmit electricity without any luminous appearance, provided they are perfectly continuous; but if they are separated in the slightest degree, a spark will occur at every separation. On this principle, various devices are formed, by pasting a narrow band of tin foil on glass, in the required form, and cutting it across with a pen knife, where we wish sparks to appear. If an interrupted conductor of this kind be pasted round a glass tube in a spiral direction, and one end of the tube be held in the hand, and the other be presented to an electrified conductor, a brilliant line of light surrounds the tube, which has been called the spiral tube, or diamond necklace. By enclosing the spiral tube in a larger cylinder of colored glass, the sapphire, topaz, emerald, and other gems may be imitated. Words, flowers, and other complicated forms, are also procured nearly in the same manner, by a proper disposition of an interrupted line of metal, on a flat piece of glass.

737. The light of the electric spark, is not a constituent part of electricity, but arises from the sudden compression of the air, or other medium through which it passes.

It is well known, that air is capable of affording a spark by sudden compression. There is a kind of match constructed on this principle, in which a small portion of air contained in a close cylinder, being suddenly compressed by forcing down a piston, yields a spark sufficient to light a quantity of tinder at the bottom of the cylinder. VOL. II.

18

Now it is found by actual experiment, that electricity has the power of condensing air. This fact is shown by means of a small instrument called Kinnersley's Air Thermometer. It consists of

Fig. 130.

a glass tube, closed air tight at the two ends by brass caps, through each of which passes a movable wire, terminated within by a small ball. Through the lower cap is inserted a small glass tube open at both extremities, and turned upwards parallel to the cylinder. Into this tube is introduced a quantity of water sufficient to cover the bottom of the cylinder, and of course to rise a little way into the tube. The two balls being set at some distance from each other, and a spark from the Leyden Jar being passed between them, the air within is suddenly rarefied, and the water ascends in the tube, and again descends, when the explosion is over. This sudden rarefaction of a portion of air before the electric spark, must cause a sudden and powerful compression in the portions of air immediately adjacent. The immense velocity of the spark must greatly increase the resistance, and of course the force of compression. This appears to be an adequate cause for the production of the light that accompanies the electric discharge, and hence we conclude, that light is not inherent in the fluid itself. The greater density and brilliancy of the spark in condensed air, and its feebleness and diffuseness in a rarefied medium, are facts which accord well with the supposed origin; and the zigzag form of the spark when long, or when passing through condensed air, is well explained by the same theory. For the electric fluid in its passage through the air, condenses the air before it, and thus meets with a resistance which turns it off laterally; in this direction it is again condensed, and has its course again changed; and so on, until it reaches the conductor towards which it is aiming. The zigzag form of lightning is accounted for on this principle. *

Electrical light is found by optical experiments, to have precisely the same nature with the light of the sun, being like this resolved into various colors by the prism, and possessing other properties, to be described under the head of Optics, which identify it with solar light.

* Biot, Traité de Phys. tome 2.-Encyc. Metropol. Art. Electricity.

CHAPTER V.

OF THE ELECTRIC BATTERY.-MECHANICAL AND CHEMICAL AGENCIES, AND MOTIONS OF ELECTRICITY.—EFFECTS OF ELECTRICITY ON ANIMALS.

738. An electric battery consists of a number of Leyden jars so combined, that the whole may be either charged or discharged at once.

any

Very large jars cannot be obtained; it is rare to find one more than two feet high, by one and a half in diameter. Yet some of the mechanical effects of electricity, to be described hereafter, require a much greater accumulation of the fluid than can be obtained from single jar. The battery is constructed as follows. Large jars, twelve or fourteen inches high, by five or six inches in diameter, are coated like ordinary Leyden jars. Twelve of these constitute a battery sufficiently powerful for most purposes, but the power of the battery may be carried to an indefinite extent by increasing the number of jars. When the number is twelve, they are placed four in a row in a box, the bottom of which is coated with tin-foil, by means of which the outsides of the jars are all in conducting communication. Each jar is separated from the rest by a slight partition of wood. To connect the insides of the jars, their knobs are joined by large brass wires. It is obvious, therefore, that the battery is equivalent to a single jar of enormous size, comprehending the same number of square feet.

The object of the battery is to accumulate a great quantity of the electric fluid, which is in proportion to the extent of surface: the intensity, or elastic force, as indicated by the quadrant electrometer, is no greater in the battery when charged, than in a single charged jar. The battery, like the common jar, is charged by bringing the inside into communication with the prime conductor of an active and powerful electrical machine:* it is discharged, as usual, by forming

* As the process of charging a large battery is tedious and laborious, it has been proposed to charge each jar successively, after that