273 



LIGHTNING. 



LIGHTNING. 



274 



!c sensation of uneasiness. Lightning flashes are now per- 

 ceived at short intervals ; their course is sometimes zigzag, when it is 

 called forked lightning ; the aberrations in its course show that it is 

 near terrestrial objects, and is therefore justly regarded as dangerous. 

 In a few seconds after the discharge, heavy showers of rain or hail 

 descend, and the atmosphere is again cooled. The blackness now be- 

 comes universal ; the thunder, which ^before roared continuously and 

 at a sensible interval after the discharge of the lightning, is now heard 

 in a loud and sudden clap almost at the same instant that the lightning 

 is seen descending towards the earth with immense velocity, and re- 

 sembling a globe of flame. 



These phenomena are the most common concomitants of thunder- 

 storms, particularly in summer time. But storms are also produced 

 by rapid changes in the atmospheric currents, for instance when the 

 equinoctial gales usually set in ; or, as met with in violent and destruc- 

 tive gales, as in crossing the Atlantic with both a revolving and pro- 

 gressive movement, and becoming mixed with various strata of the air 

 through the regions which they traverse, accompanied in several places 

 by destructive thunder-storms. 



The colour of the lightning is generally a variable yellow, but it 

 may also be red, blue, or violet, depending much on the density and 

 composition of the strata of ah- through which the discharge takes 

 place. 



In 1 752, Franklin in America, and Dalibard in France, commenced, 

 independently of each other, a series of experiments tending to identify 

 lightning with the discharge of ordinary] electricity. [ELECTRICITY, 

 ATMOSPHERIC.] Their identity might well be suspected from the 

 number of analogies known to exist between them. For example, the 

 zigzag path of the electric spark from the prime conductor of an 

 electrical machine to a conductor held within striking distance of it 

 resembles on a small scale the course of forked lightning ; both are 

 drawn off by pointed bodies in preference to others, and lightning also 

 prefers, ctttcris parlous, the best electrical conductors : both can fuse 

 metals and inflame combustibles, destroy sight and animal life, magne- 

 tise steel and reverse the poles of a magnet. 



Franklin, in June, 1752, perceiving a thunder-cloud approaching, 

 sent up a silk kite attached to a dry hempen cord. The loose threads 

 of the cord stood erect, and upon pointing his finger to the cord, he drew 

 sparks. When in consequence of a little rain the hempen cord became 

 a better conductor, the supply of electricity from the cloud became 

 more copious, and by the smartness of tiie shock ensuing, the danger 

 of prolonging the experiment was sufficiently indicated. 



A similar result had already been obtained in France a month earlier, 

 by Messrs. Dalibard and Delor, acting however in obedience to the 

 instructions sent to Europe by Franklin, in 1749. Atmospheric 

 electricity became a favourite study now that the identity between 

 lightning and ordinary electricity was established. It received a check 

 however in consequence of an accident to Professor Richmann, of St. 

 Petersburg, who having attached a simple species of electrometer to 

 his apparatus for measuring the electric intensity of a thunder-cloud, 

 proceeded, immediately after a loud clap, to read off the indication of 

 his instrument, when a globe of electric fire was discharged through 

 his body, and he fell instantly dead. This was on the 6th August, 

 1753. 



Some of the causes which produce atmospheric electricity are stated 

 under ELECTRICITY, ATMOSPHERIC. It will there be seen that the 

 atmosphere is nearly always in a different electrical condition to the 

 earth, the one being usually charged with positive electricity and the 

 surface of the earth with negative. It was shown also that the electrical 

 intensity increases with the height, that in stormy weather, in rain, 

 hail, or snow, the electricity of the air is much more intense than at 

 other times, and is liable to sudden changes from + to and from 

 to + . Now the effect of heat, evaporation, and other causes, is to exalt 

 the electrical condition of the air, while a surface of cloud acts strongly 

 by induction upon another opposed surface of cloud, or upon the 

 opposed surface of the earth beneath, and the two opposed surfaces 

 mutually reinforce each other. The cloud and the earth may thus 

 be regarded as the terminal planes of a highly charged system of 

 dielectric air, the tension of which goes on increasing until any further 

 increase causes it to break down, when the opposite electricities rush 

 together with violence, producing what is called a disruptive discharge, 

 or in other words a flash of lightning, accompanied by thunder. 

 During a thunder-storm the air between these two terminal planes may 

 be charged and discharged a number of tunes until equilibrium is 

 attained, or until the thunder-clouds are borne away by the wind to 

 another locality. 



There are various kinds- of lightning, presenting different phenomena, 

 and recognised under different names. In the first place there are 

 those narrow well-defined ribbons or lines of light, moving in a zig/ag 

 course, of a dazzling white colour, but sometimes of a violet or purple 

 hue ; these form, probably, the only kind of lightning that strikes ter- [ 

 restial objects. It proceeds from a single point, and frequently divide* 

 into two or more distinct streams. It is this bifurcation, or trifurca- 

 tion,that has obtained for it the name of forked light/tiny. When form- ! 

 ing a long rippling line of light, Bailors call it chain lightning. Some 

 admirable illustrations of this, the first kind of lightning, are given 

 under BREATH FIGURES. When the lightning appears to be spread 

 over an extensive surface, and varies in colour, being often red, but 



ARTS ASD SCI. DIV. VOL. V. 



sometimes blue and violet, often confined to the edges of clouds, but 

 when apparently issuing from the interior, the clouds are said to open, 

 we have the variety known as sheet lightning. Lightning of this kind 

 when unaccompanied by thunder is known as summer lightning, and is 

 the reflection in the upper regions of the atmosphere of common 

 lightning, having its origin in a storm, the direct view of which is 

 obstructed by the rotundity of the earth. A third kind of lightning is the 

 globular, which appears like a luminous ball or globe of fire ; it moves 

 through the air at a comparatively slow rate, while lightnings of the 

 first and second class exist but for a moment. Lightning of this kind 

 sometimes obtains the name of fire-balls : it rolls along the surface 

 with a hissing noise, and often terminates in a disruptive discharge. 

 This kind probably arises from a kind of glow or brush discharge in 

 certain points of an excited system of air, preparatory to the more 

 general and rapid union of the electrical forces. A milder form of this 

 variety of lightning is known as St. Elmo's or St. Helmo's fire to the 

 French and Spaniards, the fires of St. Peter and St. Nicholas to the 

 Italians, while the Portuguese call it Corpos Santos, which our English 

 sailors seem to have corrupted into Comazants. It appears as tufts 

 of light ivpon the points of ships' masts, the tips of spears, bayonets ; on 

 the alpenstocks of Alpine travellers, on the tips of the fingers when the 

 outspread hand is raised, and on any other pointed bodies, during a 

 peculiarly excited state of the atmosphere, when the particles of air 

 near the bodies in question discharge themselves upon the particles im- 

 mediately above them, the electrical condition of which is less intense, 

 the effect being a luminous brush of light, accompanied by a slight 

 roaring noise. This brush discharf/e is, in the language of that excellent 

 electrician Sir W. Snow Harris, " an intermitting series of electrical 

 sparks between metal and air, but in such rapid succession as to con- 

 vey the idea of a continuous stream." In some qases the appearance of 

 the brush is changed into that of a star, as when proceeding from negative 

 or resinous electricity, when the discharge is towards the pointed con- 

 ductor, and not from it as in the case of the brush discharge. A kind 

 of brush discharge usually precedes the. discharge of a Leyden jar, 

 or of a thunder-cloud upon a ship or building, marking out, as it 

 were, the line of least resistance for the principal discharge. [BREATH 

 FIGURES.] 



When lightning strikes the earth the resulting effects are of the most 

 varied character. It is a popular remark that the lightning-stroke is 

 succeeded by a suffocating odour, which has been compared to that 

 of burning sulphur. Scientific chemists have compared it to the odour 

 of phosphorus, or to that of nitrous acid; but it is more than probable 

 that this odour is due to the presence of OZONE, which is probably 

 generated by the action of lightning on the oxygen of the air. The 

 passage of lightning through the air causes a combination of its con- 

 stituent oxygen and nitrogen in the proportions to form nitric acid (N0 5 ). 

 It was shown by Cavendish that by passing artificial electrical dis- 

 charges through a confined portion of air this acid was formed. Liebig 

 has also shown that in seventeen specimens of rain-water, collected 

 during or immediately after thunder-storms, all contained nitric acid in 

 greater or less quantities in combination with lime or with ammonia, 

 while out of sixty specimens collected during ordinary rains, these sub- 

 stances were not found in fifty-eight of them. The formation of 

 ammonia (NH 3 ) during thunder-storms shows that lightning decom- 

 I oses a portion of water. The presence of ammoniacal salts in storm 

 rains is of great importance to vegetation, while the nitric acid of 

 the storm is probably one of the sources of the natural nitres of the 

 earth. 



The fusion of metals is one of the commonest effects of lightning. 

 There are instances of an iron chain being traversed by the lightning, 

 the effect of which was to soften the links so that they united by their 

 own weight, and the chain was converted into a rod of iron. Franklin 

 relates the case of a church struck by lightning, which passed along an 

 iron wire 20 feet long, and about the thickness of a knitting-needle, and 

 dissipated it in smoke, leaving a black trail behind. There are cases also 

 of the discharge passing down bell-wires, destroying them entirely, and 

 leaving the oxides resulting from the combustion impressed upon the 

 walls of the room in broad bands. A year or two ago, a house at Paris 

 was struck ; the lightning broke several panes of .glass in a shop front, 

 and burnt away the gilt letters of the word Caff. In all cases of this 

 kind the conductor is not of sufficient capacity to transmit the discharge, 

 so that the lightning being delayed in its progress, its astonishing 

 heating power becomes developed, and the result is the fusion and 

 oxidation of the metal. According to Harris, a copper rod three- 

 quarters of an inch in diameter, or an equal quantity of copper under 

 any other form, is sufficient to withstand the heating effect of any dis- 

 charge of lightning of which the effects have ever been recorded. The 

 wonderful heating effects of lightning are manifested in the formation 

 of vitrifications and lightning tubes, also known as fulgurites. [FULGURITE, 

 in NAT. HIST. DIV.] These are long, irregular, cylindrical bodies of 

 sand, or other siliceous material, fused into a kind of glass or enamel, 

 and appear to have been formed by the lightning striking and pene- 

 trating the rock, and fusing the loose materials together. The outer 

 surface of the tube is often rough and uneven, and deeply furrowed, 

 while the interior may be formed of whitish or limpid vitrified 

 matter covered with a smooth glaze, and hard enough to scratch glass. 



When lightning strikes solid imperfect conductors, it tears them to 

 pieces, and scatters their fragment* to great distances, just as if a 



