Chapter XXII (2)

Before concluding the subject of electricity we must devote a few pages to the consideration of the electric influence possessed by certain fishes, and to some of the phenomena of the atmosphere, especially thunderstorms. We have seen how Galvani experimented upon the limbs of frogs, and maintained that they possessed electricity; he attributed the current in the muscles to that cause. This theory Volta denied, but subsequently Nobili, in 1827, proved the existence of a current in the frog by means of a Galvanometer. This was conclusive, and the experiment was performed in the following manner:—He filled two vessels with salt and water, and into one he dipped the crucal muscles of a frog, and in the other the lumbar nerves were immersed. By putting these vessels in communication with his improved Galvanometer, which was extremely sensitive, he perceived a current passing from the feet towards the head of the animal.

It is, however, to Matteucci and Du Bois Reymond that the investigation of the phenomena of the _courant propre_ are due. The former formed a “pile” of the thighs of frogs, and by placing the interior and exterior muscles in contact he formed a current from the inside to the outside muscles. This current is supposed to be occasioned by certain chemical changes which are continually taking place, and it continues longer in the case of a cold-blooded animal than in a warm-blooded one. There are many interesting papers on this subject included amongst the “Philosophical Transactions”; and the “Physical Phenomena of Living Beings” is fully treated in Matteucci’s lectures on that subject. In the “Transactions” for 1848 and subsequent years, other experiments may be perused, but space will not permit us to dilate upon them. The fact has been established, and we are told that muscles and nerves, as well as certain glands of the body, possess certain electrical properties.

The electricity of fishes, and the power possessed by the torpedo—whose name is now chiefly known in connection with warlike appliances—and the gymnotus, have been known for a very long time. This fish, popularly known as the electric eel, inhabits the warm fresh-water lakes of Africa, Asia, and America. A specimen was exhibited at the Polytechnic some years ago. This was the fish experimented on at the Adelaide Gallery by Professor Faraday, who clearly demonstrated the fact that the electricity of the animal and the common electricity are identical. Numerous experiments were made, and the circuit shock and even sparks were obtained from the gymnotus. In fact, the gymnotus is a natural electric machine. The force of the shock given by the electric eel is very great, for Faraday has put on record that a single discharge of the eel is equal to fifteen Leyden jars charged as highly as possible. Its power does not even end there, for having shocked people to that extent, it was capable of a second and occasionally of a third shock of less violence.

The manner in which the gymnotus acts is from a regular battery in the head, the sides of which are filled with a fluid. These cells are something like a honeycomb in appearance. The shock is quite voluntary on the part of the fish. Sometimes it will kill its prey, on other occasions it is merely numbed. Professor Faraday on one occasion placed a live fish in the tub with the gymnotus, which curled itself so as to enclose the unsuspecting one. In a second the prey was struck dead, and floated on the water. The gymnotus immediately devoured it, and went in quest of more. Another, but an injured fish, was then introduced, but the electric eel took no trouble about this one. It did not trouble to give it a shock, seeing it was disabled, it merely swallowed without killing it. It is also on record that on one occasion an electric eel had stunned a fish which, before he began to eat it, gave signs of returning animation; the eel immediately gave it another shock and killed it.

There were some other curious peculiarities connected with the electric eel. It appears to be quite capable of discriminating between animate and inanimate touch. For instance, when touched with a glass rod it at first gave signs of electricity, and discharged a shock at the attacking party. But on subsequent occasions, when touched with metal rods or glass, the fish declined to “shock”; nevertheless the Professor succeeded the moment he touched the animal with his hands.

The torpedo is something like the well-known skate; it is sometimes called the electric ray, and is common enough in the Bay of Biscay and in the Mediterranean Sea. It sometimes pays England a visit, or is caught by fishermen and brought in. We have seen one at Plymouth, and a very ugly-looking fish it was. Its electric power is considerable.

_c_, brain; _m_ _e_, spinal chord; _o_, eye; _e_, electric organs; _b_, gills; _np_, _nl_, nerves; _n_, spinal nerve.]

There is yet another fish known as the malapterurus; one species is called the thunder-fish. Professor Wilson has written a paper upon the electric fishes as applied to the remedy of disease, and considers them the “earliest electric machines ever known.”

Humboldt relates that the South-American Indians capture the gymnotus by driving horses into ponds which the electric eels are known to inhabit. The result is that the fish deliver shock after shock upon the unfortunate quadrupeds. Mules and horses have frequently been killed by these powerful eels, and even Faraday experienced a very great shock when he touched the head and tail of the captive gymnotus with either hand.

The malapterurus to which we have referred is an inhabitant of the African rivers, chiefly in the Nile and Senegal. Such a fish has been known with others for some hundreds of years; its electrical powers are not great. There are one or two other species of fish which possess electrical qualities, but none apparently to the same extent as the torpedo and the gymnotus.

The electricity of plants also is in some cases very marked. Flashes have been seen to come from some flowers in hot and dry weather. Currents of electricity have been detected, and Wartmann investigated the subject closely. He says the currents in flowers are feeble, but in succulent fruits and some kinds of grain they are very marked. These currents depend upon the season, and are greatest in the spring, when the plant is bathed in sap. These experiences were confirmed by Bequerel in 1850, and he concludes that the rank vegetation in some parts of the world must exercise considerable influence on the electric phenomena of the atmosphere. M. Buff has more recently made experiments in this direction, and he examined plants and trees, and even mushrooms. M. de la Rive, after carefully summing up the various theories, comes to the conclusion that it is to chemical reactions that the traces of electricity are due.

The subject of atmospherical electricity properly belongs to meteorology, and under that heading we will treat of it more fully. But lightning is so identified with electricity, and being the most common form observable to all, we will say something about thunderstorms and the electric discharges accompanying them.

Before Franklin’s ever-memorable experiment with his kite established the identity of lightning and electricity, the resemblance between the two discharges had been frequently noticed. The Etruscans pretended to bring down lightning from heaven, and Tullus Hostilius, when experimenting or performing certain “ceremonies,” was killed by the electric discharge he desired to attract. But after all, we cannot attribute any knowledge of electric science to the ancients, although they were, of course, familiar with electric phenomena.

It is to Dr. Wall that testimony points as the first person who remarked the analogy between the electric spark and lightning. This was in 1708. Grey and other philosophers supported the theory, but could not establish it. To Franklin, who in June 1752 actually brought down the lightning by his kite and a key, is the actual discovery due. We have already detailed the circumstances (page 206) and need not repeat the account of the experiment.

Of course the American philosopher found numerous imitators, not always with impunity. Professor Richmann was killed by the spirit he was invoking; Lemounier and Beccaria confirmed the theory that the air was full of electricity; while Du Saussure, from his investigations on the Alps, and Volta from the invention of the pile, are most famous in the history of electricity. They applied themselves with much success to the investigation of the electric condition of the atmosphere, of which the disturbances called thunderstorms are the result.

The amount of electricity varies in the atmosphere at different times in the day and night. Towards midday and midnight the development is generally greatest, and this fact will account for the prevalence of storms during our hours of rest. Again, different kinds of clouds have different degrees of electricity, and of different kinds. Under certain conditions these clouds will give forth lightning, and a storm will begin. The more clouds the more globules, and therefore in summer, while there is more production of vapour from solutions of salts, etc., we are more likely to have the storms. We are most of us familiar with the mass of the “thunder cloud” rising in the distance, light at the upper part, very dark below, and throwing out tentacles like the octopus, coming up sometimes—frequently, indeed—“against the wind,” impelled by an upper current, or following the course of a river, which is not unusual. Below, there is perhaps an army of thin dark clouds. The nature and height of clouds have also a great deal to do with the phenomena displayed. In general, storm-clouds are positively electrified.

Clouds are good conductors of electricity, and yet they may be so insulated by the dry air surrounding them that they will accumulate it; and when thus charged, if they encounter other clouds charged with opposite electricity, the opposing masses will attract each other until a discharge takes place. This is what we term lightning, and under such conditions electricity, though very dazzling, is harmless. It is when the cloud comes near to the earth, and a discharge is released, that lightning is so dangerous to persons who remain in the fields. Sometimes the discharge comes from the earth to meet that from the cloud. Sheep are frequently killed by ground lightning, and once, at Malvern, we had an escape from an upward stroke. The back-stroke from a cloud is also dangerous. It may happen that the cloud has discharged itself upon the earth many miles away, but a return discharge takes place at the other end, and if that end be near the earth the consequences may be serious. As a rule, the return stroke is not so violent as the first discharge.

The colour of lightning varies very much. We have the white, the blue, the violet, and red. The colour depends upon the distance and intensity of the lightning, and the more there is of it the whiter the light. We can illustrate the varied hues of the electric “fluid” by passing a spark through the receiver of an air-pump. If the air be rarefied, or there be a vacuum, we shall perceive a blue or violet light. Therefore we may conclude that the blue and violet flashes have birth in high strata of the atmosphere.

We have all heard how dangerous it is to stand under a tree during a thunderstorm, or rather, we should say, when the storm is approaching us nearly. The tree is a conductor, and the lightning having no better one at hand will pass through the tree on its way to the earth, and if we are standing against the tree we shall be included in the course, and die from the shock to the nerves while the lightning is passing through us. The best position in a thunderstorm, if we are in the neighbourhood of trees, is to sit or lie down on the ground some little distance from the base of the nearest tree. If the tree be sixty feet high suppose, and we sit fifty feet or less from the trunk, we shall be pretty safe, because the lightning will reach the tree top before it can reach us. We are protected by it as by a conductor, bad though it be. Standing up in a boat during a storm is not wise. Lightning has an affinity for water, and besides, if no higher objects are near, our body will act as a conductor. Bed is the safest place, as blankets are non-conductors. Cellars are not the safest by any means. Lightning may, and it frequently does, strike the house and descend to the basement. If the air be very full of electricity, and a flash be near, a person running away may conduct the lightning to himself by creating a vacuum into which the flash may dart.

Arago classified lightning into three kinds: zig-zag, globular, and sheet. The first we call forked lightning, and frequently this kind branches out at the end, so that although there may be only one flash, it may strike out in two or three directions at the same time. This may be accounted for by the unequal power of the air strata to conduct the electricity. The forked flashes are of very great length, extending frequently for miles, and the bifurcations also are often miles apart. The duration of the flash is less than the thousandth part of a second; so instantaneous is it that no motion can be perceived even in a most rapidly-moving wheel, as proved by Professor Wheatstone. We sometimes fancy that the flash lasts longer, but the impression received by the eye quite accounts for the apparently prolonged sight of the lightning.

Sheet lightning, the faint flashes frequently seen upon the horizon, are quite harmless. Sheet lightning is that which is seen reflected behind clouds or from far-distant storms. It is sometimes very beautiful. Ball, or globular lightning, is dangerous, and globes of fire have been seen to descend, and striking the ground, bound onwards for some distance. The descent of these forms of electric discharge has given rise to the popular notion of “thunderbolts.” The “Mariner’s Lights,” or St. Elmo’s fire, is frequently observed in ships. It is usually regarded as a fortunate occurrence. It was noticed by Columbus. One voyager describes the phenomena as follows:—“The sky was suddenly covered with thick clouds.... There were more than thirty of St. Elmo’s fires on the ship. One of them occupied the vane of the mainmast. I sent a sailor to fetch it. When he was aloft he heard a noise like that which is made when moist gunpowder is burned. I ordered him to take off the vane. He had scarcely executed this order, when the fire quitted it and placed itself at the apex of the mainmast, whence it could not possibly be removed.”

There have been occasions when the manes and tails of horses, and even the ears of human beings, have shown a phosphorescent light which emitted a hissing noise. Alpine travellers have noticed similar phenomena; and Professor Forbes, when crossing the Theodule Pass into Italy, heard the hissing sound in his alpenstock. The tips of rocks and grass points were all hissing too. The party were in the midst of an electric cloud. When the Professor turned the point of his alpenstock upwards, a vivid flash was emitted, but no thunder followed. They descended as quickly as possible from such a dangerous neighbourhood.

It is observable that the properties of lightning and of the electric spark are identical—the faint crackle of the latter being magnified into the loud rolling of the thunder. The disturbance of the atmosphere is the cause of the loud reverberations, and echoes produced from clouds tend to intensify and prolong the peal. The sound rises and falls, and varies accordingly as the cloud is near or far. A smart sharp report or rattle denotes the nearness of the lightning, while the gradual swelling and subsidence, followed, mayhap, by an increasing volume of sound, which in its turn dies away, tells us that the danger is not imminent. The cause o£ this loud rolling, unless it proceeds from echoes from different clouds, has not been satisfactorily explained. Sound travels less quickly than light, and therefore we only hear the thunder some seconds after we have perceived the flash. It is therefore conceivable that we may hear the last reverberations and its echoes first, and the sound of the first disturbance with its echoes last of all. Thus there will be distinct sounds. Firstly, the actual noise we call thunder from the air strata _nearest_ to us; secondly, the echoes of that disturbance from the clouds, of course fainter; then we hear the sound caused by the tearing asunder of the air particles farthest off, and again the echoes of that disturbance. This theory will, we think, account for the swelling peals of thunder, and the successive loud and fainter reverberations. At any rate, in the absence of any other theory, we submit it for consideration. The sound of thunder is seldom or never heard at a distance greater than fifteen miles.

Lightning conductors are such every-day objects that no description is necessary; but the reason the lightning runs along it harmlessly is because the galvanized iron rod is the best conductor in the immediate neighbourhood. Where there is not a good conductor lightning will accept the next best, and so on, any conductor being better than none. The point of the rod cannot contain any electricity, there being no room for it, and the “fluid,” as it is termed, runs down to the ground, to terminate, when possible, in water or charcoal. A great deal of electricity is no doubt carried away from the air by the numerous conductors without any spark passing. Until Sir W. Snow Harris brought forward his lightning conductors for ships, the loss was great at sea. But now we rarely hear of any vessel being disabled by lightning. We owe to Franklin the idea of the lightning conductor.

According to observations made by Mr. Crosse, the following statement shows the tendency of the atmosphere, in certain conditions, to thunderstorms. We may accept the deduction of M. Peltier that grey and slate-colour clouds are charged with negative, and white, rose-colour, and orange clouds with positive electricity. The order of arrangement in the following table places the most likely source of thunderstorms first, and the least likely source at the end, with regular rotation of intermediate probabilities intervening:—

1. Regular thunder clouds.
2. Driving fog with small rain.
3. Fall of snow, or hailstorm.
4. Smart shower on a hot day.
5. Smart shower on a cold day.
6. Hot weather after wet days.
7. Wet weather after dry days.
8. Clear frosty weather.
9. Clear warm weather.
10. Cloudy days.
11. “Mackerel” sky.
12. Sultry weather and hazy clouds.
13. Cold damp night.
14. Cold, dry north-east winds.

We have thus briefly touched upon some of the atmospherical phenomena directly attributable to electricity. In our articles upon Meteorology we will consider the aurora and many other interesting facts concerning the atmosphere, and the effects of sound, heat, and light upon the air.

FOOTNOTES:

[18] We are indebted for many facts respecting Mr. Edison’s light in this chapter to a paper by Mr. Upton.