ELECTRICITY. 



439 



Desciiptive the charged jar under the receiver of an air pump, and 



Kkctricity. tx ] laus t the receiver. The electric light will then flow 



""" "V" ou t copiously from the neck of the bottle, and will divide 



itself into several jets, wliich will bend round in order 



to reach the internal surface of the jar. If the jar is 



cliarged 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. 



Exp. 11. Into a piece of soft deal, about three inch- 

 es long, and 1^ inch square, insert obliquely two 

 pointed wires, having their points 1^ inch distant, 

 and penetrating 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 exhibit different colours at different 

 depths. If the 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 from imperfect conductors. In the vapour 

 of ether, the green sparks appear while the eye is 

 placed close to the tube, and reddish when 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, t 

 Upon rarefying the air 500 times within a glass jar, 

 about one foot long and eight inches in diameter, Mr 

 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 ap- 

 peared 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 conducting power. When 

 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 be- 

 fore 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, coming from the knob of the 

 jar, at the distance of a quarter of an inch from the for- 

 mer, and set a glass vessel full of water upon both these 

 ends. When the jar is discharged, the water will ap- 

 pear completely luminous. 



Exp. 1 3. 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 still more brilliantly 

 illuminated when the charge of a battery is passed 

 through it 



Desp-iptiw 



Electru-.M. 



Luminous 

 spiral tubes. 



PLATE 

 CCXLIII. 

 Fig. 18. 



Fig. 1. 



Exp. 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 l-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 illuminate it in such a manner that the bone and 

 the principal blood vessels may be readily observed. 



Exp. 15. Place a lump of sugar on the table, and hi 

 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- 

 minous, and retains the light for a considerable time. 



Exp. 16. Paste upon a tube of glass a number of small 

 round pieces of tinfoil, as in Fig. 1 8. the distance be- 

 tween each being about the 30th of an inch. When 

 this tube is held near the prime conductor, a brilliant 

 line of light will appear to surround it, a spark being 

 visible between each of the small circles of tinfoil. 



Eip. 17- Let several of those spiral tubes be placed 

 on a board, as in Fig. 19- and round a central glass pil- 

 lar supporting a wire a i, having a ball a and b at each ex- 

 tremity, and capable of turning round an axis. Com- 

 municate several sparks from a conductor to the ball C, 

 and pushing the wire a l> gently round, each tube will 

 receive a spark from the balls, and will be illuminated 

 as in the preceding experiment. 



In like manner, luminous words may be made, as in 

 Fig. 20. and a similar effect may be produced, by dis- 

 charging a jar coated in the manner represented in Fig. 

 21. where there are on the outside five rows of square 

 pieces of tinfoil, with eleven in each row, and the same 

 number on th inside of the jar, making in all 110 square 

 pieces. The diagonals of the square pieces are in a ho- 

 rizontal and a vertical position, and there is a round hole 

 in the centre of each. Every adjacent pair on the out- 

 side jar, have two- of their square points meeting in the 

 centre of the round hole in the intervening pieces of 

 tinfoil on the inside, so that 110 sparks will be seen at 

 once in a horizontal direction upon the discharge of the 

 jar, and 110 sparks in a vertical direction. 



It was imagined by some of the early electricians, 

 that the electric light contained no prismatic colours. 

 Dr Priestley observed, however, that this was a mis- 

 take, and a similar result has been more recently ob- 

 tained by Dr Wollaston. J In every respect indeed, the 

 light of the electric spark comports itself like the direct 

 light of the sun, or of a candle. The writer of this ar- 

 ticle has found, that it is capable of being polarised like 

 other lights, either by transmission through a doubly 

 refracting crystal, or by reflexion at the polarising 

 angle from the surface of a transparent body, or by 

 oblique refraction through a bundle of glass plates. 



SECT. VI 1 1. On the Influence of Points in receiving and 

 discharging Electricity. 



IN the Section upon electric light, we have already on the in- 

 seen the effect of obtuse bodies, such as balls, in carrying fluence of 

 off the electricity of a conductor by a succession of bril- points in re- 

 liant sparks; and we had occasion to observe, that when "iving an<1 

 a ball was thus receiving sparks, any pointed body pla- 



Luminous 

 words. 



Fig. 80. 



Luminous 

 jar. 



Fig. 81. 



Electric 

 light has tlie 

 same opti- 

 cal proper- 

 tics as any 

 other lighu 



See Morgan's Lecture*, p. 2S4, and Singers' Element! of Electricity, p. 91, 91. 



f Morgan. Phil. Traiu. 1785, voL Ixxv. p. 873. 



* See Priestlty'j Uiitory of Electricity, p. 125, and Phil. Tram. 1808. 



