In condensed air and in carbonic acid gas, the light of the spark is white and brilliant, but in hydrogen gas it is red and faint.
Ealz . 10. Charge a Leyden phial by its knob, and take away the knob by a stick of sealing wax. Place the charged jar tinder the receiver of an air pump, and exhaust the receiver. The electric light will then flow out copiously from the neck of the bottle, and will divide itself into several jets, which will bend round in order to reach the internal surface of the jar. If the jar is charged by its external coating. and placed under the receiver as before, the electric light will throw itself in jets, from the external coating to the neck of the jar.
Eap.1 1. Into a piece of soft deal, about three inches long, and 1 inch square, insert obliquely two pointed wires, having their points I inch distant, and penetrat ing to different depths below the surface, but so that the line joining them is in the direction of the fibres. While the spark passes from the one wire to the other, it will ex hibit different colours at different depths. lithe one point is deeper than the other, all the colours will appear at once, as the electric light is transmitted at various depths.
When the sparks pass through balls of wood or ivory, they are of a crimson colour. They are green when taken from the surface of silvered leather, yellow when taken over powdered charcoal, and of a purple colour when taken front imperfect conductors. In the vapour of ether, the green sparks appear while the eye is placed close to the tube, and reddish seen at a considerable distance.* The colour of the spark in very rare air is green, and in denser air it becomes blue, and then violet and purple, till the medium has become so dense as to be unable to conduct electricity 4 Upon rarefying the air 500 times within a glass jar, about one foot long and eight inches in diameter, Air Smeaton placed the jar upon a lathe, and turned it round rapidly, whilst at the same time he rubbed it with his hand. A considerable quantity of lambent flame appeared under his hand, variegated with all the colours of the rainbow. The light was steady, but every part of it was constantly changing colours.
When metallic substances are completely continuous, and of sufficient size, they do not become luminous by the transmission of the electric light. The difficulty therefore of making bodies luminous by electricity, must increase with their power. \Vhen there is any breach of continuity in the body, a spark will always appear at every separation ; hence at each link of an electrified chain, a spark is distinctly visible, and, from a similar cause, perhaps, gold leaf may be ren dered luminous. The appearance of light at the sepa ration of the parts of a metallic body, is the foundation of several curious experiments, which we shall lay before our readers.
Exp. 12. Having connected one end of a chain with the outside coating of a charged phial, place the end of another piece of chain, coining from the knob of the jar, at the distance of a quarter of an inch from the former, and set a glass vessel full of water upon both these ends. When the jar is discharged, the water will appear com pletely luminous.
Esp. 13. A strip of gold leaf, about the eighth of an inch broad, and three feet long, will be rendered lumi nous throughout its whole extent by the explosion of a jar containing two gallons. if the gold leaf is laid on glass, and below water, it will be s'.;11 more brilliantly illuminated when the charge of a battery is passed throuei it.
E.7:1 14. Attach one wire to the outside coating of a charged jar, and another to one of the branches of the discharging rod, and having placed their other ends at the distance of 1-10th of an inch, press the thumb upon the two ends. Bring the other branch of the discharging rod in contact with the knob of the jar, which being thus discharged, the spark will pass under the thumb, and il luminate it in such a manner, that the bone and the prin cipal blood vessels may he readily observed.
Exp. 15. Place a lump of sugar on the table, and in contact with the outer coating of a charged jar. Place one of the knobs of the discharging rod on the outside of the sugar, and bring the other to the knob of the jar. As the spark passes through the sugar it is rendered lu minions, and retains the light for a considerable time.