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Thus, if a piece of wire net-work, with meshes sufficiently close, be held over the flame of a lamp or candle, it will be found that the flame will not pass through the meshes. The wire will become red hot, but no flame will appear above it.
It is not, in this case, that the gas which forms the flame does not pass through the meshes of the wire, but in doing so, it gives up so much of its heat to the metal, that when it escapes from the meshes above the wire, it is no longer hot enough to be luminous.
Sir Humphrey Davy, in the researches which he was called to make, discovered this important fact, which enabled him to explain the nature and properties of flame; and having so discovered it, he did not fail promptly to apply it to the solution of the practical problem with which he had to grapple.
or candle, instead of being as usual enclosed by glass or horn, was enclosed by wire gauze of that degree of fineness in its meshes which experiment had proved to be impervious to flame. When such a lantern was carried into an atmosphere of explosive gas, the external atmosphere would enter freely through the wire gauze, and would burn quietly within the lantern; but the meshes which thus permitted the cold gas to enter, forbid the white-hot gas within to escape without parting with so much of its heat in the transit as to deprive it of the character and properties of flame; so that, although it passed into the external explosive atmosphere, it was no longer in a condition to inflame it.
The lamp thus serves a double purpurpose it is at once a protection and a warning. It protects, because the flame within cannot ignite the gas outside the lantern. It warns, because the miner, seeing the gas burning within the lantern, is informed that he is enveloped by an explosive atmosphere, and takes measures accordingly to ventilate the gallery, and meanwhile to prevent unguarded lights from entering it.
Nothing can be imagined more triumphantly successful than this investigation of Sir Humphrey Davy. Some philosophers have the good fortune to arrive at great scientific discoveries in the prosecution of those researches to which the course of their labours leads them. Some are so happy as to make inventions of high importance in the arts, when such applications are suggested by the laws which govern the phenomena that have arisen in their experimental researches. But we cannot remember any other instance in which an object of research being proposed to an experimental philosopher, foreign to his habitual inquiries, having no associations with those trains of thought in which his mind has been previously involved, he has prosecuted the inquiry so as to arrive not only at the development of a natural law of the highest order, the fruitful parent of innumerable consequences of great general importance in physics, but has at the same time realised an invention of such immense utility as to form an epoch in the history of art, and to become the means of saving countless numbers of human lives.
As wire-gauze drains flame of its danger in the safety-lamp, it drains air of its poison by another felicitous application of a physical principle in the case of the needle-grinder's mask. In that department of industry, the health of the artisan was impaired, and the duration of his life abridged, by respiring continually, while at work, an atmosphere impregnated with steeldust. A mask was invented composed of a gauze formed of magnetised wire, through which the artisan was to breathe. The air, in passing from the external atmosphere to the mouth and nostrils, left all the steel-dust which it held in suspension on the wire of the mask, from which, from time to time, it was wiped off as it accumulated.
Electricity has proved a fertile source of benefits conferred on Art by Science. When a galvanic current is passed through a fluid which holds in solution any substance which has the property of being attracted by one of the poles of the battery, such substance will desert the fluid, and collect upon any object, being a conductor, which may be used to form the attracting-pole.
This fact has been already variously applied in the arts, and in no case with greater felicity and success than in the process of gilding and silvering the
The process of electro-gilding or plating, which now forms so important a department of industrial art, is easily described.
Let us suppose that it be required to gild an object formed of silver, copper, or any inferior metal. The object, being first fabricated in the form it is destined to have, is submerged in a fluid which holds gold in solution. It is then put in connexion with the attracting pole of the galvanic battery, while the solution of gold is put in connexion with the other pole. The galvanic current thus passing through the solution, will decompose it, and the gold will attach itself to the metallic object, which in a few seconds will be sensibly gilt.
Any quantity of gold which may be desired can thus be deposited on the surface of the object. This is accomplished merely by allowing it to remain for a longer period of time in the solution. Thus the gilding may be regulated with the utmost precision, and the quantity of gold which has been
deposited over the object to be gilt may always be known with perfect exactitude.
An object may be silvered in some parts and gilt in others, by a very simple expedient. Let the parts intended to be gilt be coated with some nonconducting substance not affected by the solution of silver, and let the object be then immersed in the solution, and put in connexion with the galvanic battery as already described. The parts not coated will then be plated. Let the parts thus plated be now coated with a non-conducting substance not affected by the solution of gold, the coating previously applied being removed, and let the object be immersed in the solution of gold, and being connected with the battery, the parts not coated will be gilt.
The result of the two operations will be, that the object will be plated on some parts and gilt on others.
In this manner, beautiful effects are produced on vessels used for domestic purposes, which are adorned with various designs expressed by such combinations of plating and gilding.
But of all the applications of electric agency to the uses of life, that which is transcendently the most admirable in its effects, and the most important in its consequences, is the electric telegraph. No force of habit, however long continued, no degree of familiarity can efface the sense of wonder which the effects of this most marvellous application of science excites. If any sanguine and far-seeing votary of science had ventured thirty years ago to prognosticate the events which are now daily and hourly witnessed in the Central Electric Telegraph Office, Lothbury, at the Ministry of the Interior in Paris, or in the Telegraphic Bureau at New York, he would have been pronounced insane by every soberminded and calmly-judging person.
It is not many weeks since we, being in Paris, entered the Telegraphic Office, at the Ministry of the Interior, in the Rue Grenelle St. Germain. There we found ourselves in a room about twenty feet square, in the presence of some half-dozen persons seated at desks, employed in transmitting to, and receiving from various distant points of France, despatches. Being invited, we dictated a message, consisting of about forty
words, addressed to one of the clerks at the railway-station at Valenciennes, a distance of an hundred and sixtyeight miles from Paris. This message was transmitted in two minutes and an half. An interval of about five minutes elapsed, during which, as it afterwards appeared, the clerk to whom the message was addressed was sent for. At the expiration of this interval the telegraph began to express the answer, which, consisting of about thirty-five words, was delivered and written out by the agent at the desk, in my presence, in two minutes. Thus, forty words were sent an hundred and sixty-eight miles, and thirty-five words returned from the same distance, in the short space of four minutes and thirty seconds.
miles, and were joined to the extremity of the wire returning from Lille, thus making one continued wire measuring one thousand and eighty-two miles. A message consisting of two hundred and eighty-two words was now transmitted from one end of the wire. A pen attached to the other end immediately began to write the message on a sheet of paper, moved under it by a simple mechanism, and the entire message was written in full in the presence of the Committee, each word being spelled completely and without abridgement, in fifty-two seconds, being at the average rate of two words and four-tenths per second!
By this instrument, therefore, it is practicable to transmit intelligence to a distance of upwards of a thousand miles, at the rate of nineteen thousand five hundred words per hour!
But surprising as this was, we soon afterwards witnessed, in the same room, a still more marvellous performance. A memoir on an improvement on the Electric Telegraph, by Mr. Alexander Bain, having been read before the Institute, and submitted to the Committee of the Legislative Assembly appointed to report on the project of law for opening the telegraphs to the use of the public, a series of experiments were ordered to be made, with the purpose of testing this alleged improvement. The Committee, among whom were M. Leverrier (celebrated for having discovered a planet before it was visible), M. Pouillet, professor of physics, and other distinguished persons desiring to submit the invention to a more severe test as to distance, than the existing telegraphs supplied the means of accomplishing, adopted the following expedient:-Two telegraphic wires, extending from the Ministry of the Interior to Lille, were united at the latter place, so as to form one continuous wire, extending from the Ministry to Lille, and back from Lille to the Ministry, making a total distance of three hundred and thirty-six miles. This, however, not being deemed sufficient for the purpose, several spiral coils of wire, wrapped in silk, were obtained, measuring in their total length seven hundred and forty-six
The instrument would, therefore, transmit to a distance of a thousand miles, in the space of an hour, the contents of twenty-six pages of the book now in the hands of the reader!!
But it must not be imagined, because we have here produced an example of the transmission of a despatch to a distance of a thousand miles, that any augmentation of that distance could cause any delay of practical importance. Assuming the common estimate of the velocity of electricity, the time which actually elapsed in the transition of the despatch in this case was the two-hundredth part of a a second. If, therefore, instead of sending the despatch along a thousand miles of wire, we had sent it along a wire completely surrounding the globe, the time of its transmission would still be only the eighth part of a second.*
Such a despatch would fly eight times round the earth between the two beats of a common clock, and would be written in full at the place of its destination more rapidly than it could be repeated by word of mouth. When such statements are made do we not feel disposed to exclaim
"Are such things here as we do speak about? Or have we eaten of the insane root,
That makes the reason prisoner."
The wildest flights of the most exalted
We have here taken the usual estimate of the speed of an electric fluid; récent experiments render it probable that it is somewhat less, and depends on the conductibility of the wire. Thus copper and iron give different rates of transmission,
The apparatus used for producing the electric fluid consists of a series of plates of zinc and copper, united in pairs, and placed in a porcelain, or wooden trough. The zinc plates are previously rubbed with mercury, which, combining with the superficial part of the zinc, forms a coating of amalgam, which renders the development of the electricity more regular and uniform. The cells between the successive pairs of plates are filled with dry and perfectly clean sand, which is moistened with a solution consisting of eleven parts of water to one of strong sulphuric acid.*
A series of troughs, thus arranged, are called a galvanic battery; and if they be united by metallic connexionsthe series of plates following the same order, and their extremities being connected by a metallic bar or wire-a
continuous current of electricity will be propagated along such bar or wire, from one end of the battery to the other. Batteries of this kind are simple, cheap, steady, and continuous in their effects; their action being maintained during a period of four or five months, no other attention being required than to renew the acid solution from time to time, with which the sand is moistened.
Such an apparatus as that which we have here described, is to the electric telegraph what a boiler is to a steam engine. It is the generator of the fluid by which the action of the machine is produced and maintained.
We have next to explain how the electric fluid, generated in the apparatus just explained, can be transmitted to a distance without being wasted or dissipated in any injurious degree en
If tubes or pipes could be constructed with sufficient facility and cheapness, through which the subtle fluid could flow, and which would be capable of confining it during its transit, this object would be attained. As the galvanic battery is analogous to the boiler, such tubes would be analogous in their form and functions to the steam-pipe of a steam engine.
The construction of such means of transmission has been accomplished by means of two well-known properties of the electric fluid, in virtue of which it is capable of passing freely over a certain class of bodies called conductors, while its movement is arrested by another class called non-conductors, or insulators.
The most conspicuous examples of the former class are the metals; the most remarkable of the latter being resins, wax, glass, porcelain, silk, cotton, &c., &c.
Now, if a rod or wire of metal be coated with wax, resin, silk, cotton, or other insulator, the electric fluid will pass freely along the metal, in virtue of its character of a conductor; and its escape from the metal to any lateral object will be prevented by the coating, in virtue of its character of an insulator.
The insulator in such cases is, so far as relates to the electricity, a real
Other combinations are occasionally used, but the principle is the same.
tube, inasmuch as the electric fluid passes through the metal included by the coating, in exactly the same manner as water or gas passes through the pipes which conduct it; with this difference, however, that the electric fluid moves along the wire more freely, in an almost infinite proportion, than does either water or gas in the tubes which conduct them.
it would be sufficient to connect the wires at each end with the gas or water pipes which, being conductors, would equally convey the fluid to the earth; and in this case, every_telegraphic despatch which flies to Edinburgh along the wires which border the railways, would fly back, rushing to the gas-pipes which illuminate Edinburgh--from them through the crust of the earth to the gas-pipes which illuminate London, and from them home to the batteries in the cellars at Lothbury.
If, then, a wire, coated with a nonconducting substance, capable of resisting the vicissitudes of weather, were extended between any two distant points, one end of it being attached to one of the extremities of a galvanic battery, a stream of electricity would pass along the wireprovided the other end of the wire were connected by a conductor with the other extremity of the battery.
To fulfil this last condition, it was usual, when the electric telegraphs were first erected, to have a second wire extended from the distant point back to the battery in which the electricity was generated. But it was afterwards discovered that the EARTH ITSELF was the best and by far the cheapest and most convenient conductor which could be used for this returning stream of electricity. Instead, therefore, of a second wire, the extremity of the first, at the distant point to which the current is sent, is attached to a large metallic plate, measuring five or six square feet, which is buried in the earth. A similar plate, connected with the other extremity of the battery, at the station from which the current is transmitted, is likewise buried in the earth, and it is found that the returning current finds its way back through the earth from the one buried plate to the other buried plate.
Of all the miracles of science, surely this is the most marvellous. A stream of electric fluid has its source in the cellars of the Central Electric Telegraph Office, Lothbury, London. It flows under the streets of the great metropolis, and, passing along a zigzag series of railways, reaches Edinburgh, where it dips into the earth, and diffuses itself upon the buried plate. From that it takes flight through the crust of the earth, and finds its own way back to the cellars at Lothbury!!
Instead of burying plates of metal,
The atmosphere, when dry, is a good non-conductor; but this quality is impaired when it is moist. In ordinary weather, however, the air being a suf ficiently good non-conductor, a metallic wire will, without any other insulating envelope except the air itself, conduct the stream of electricity to the necessary distances. It is true that a coated wire, such as we have already described, would be subject to less waste of the electric fluid en route; but it is more economical to provide batteries sufficiently powerful to bear this waste, than to cover such extensive lengths of wire with cotton, or any other envelope.
The manner in which the conducting wires are carried from station to station is well known. Every railway traveller is familiar with the lines of wire extended along the side of the railways, which, when numerous, have been not unaptly compared to the series of lines on which the notes of music are written, and which are the metallic wires on which invisible messages are flying continually with a speed that surpasses imagination.
These wires, in the case of the English telegraphs, are galvanised so as to resist oxydation, and are of sufficient thickness to bear the tension to which they are submitted. They are suspended on posts, erected at intervals of sixty yards, being at the rate of thirty to a mile. These posts, therefore, supply incidentally a convenient means by which a passenger can ascertain the speed of the train in which he travels. If he count the number of telegraph posts which pass his eye in two minutes, that number will express in miles per hour the speed of the train.
To each of these poles are attached as many tubes or rollers of porcelain